Rework how we create/save revisions and desired state

[romangithub1024]

We need to start saving desired state, so that:

1. We don't have to calculate it every single time
2. History of all desired states is preserved

We should also consider creating revisions on policy upload.

Need to figure out to do this efficiently in Bolt.

[romangithub1024]

Decided that:

• revisions should be created on policy update
• desired state needs to be saved for every revision
• enforcer should just enforce revisions
• every policy version should have at least one revision tied to it

[romangithub1024]

Also, taking last policy and new revision = last revision + 1 logic is currently incorrect when you have policies without any revisions.

So, e.g. when deleting a dependency this happens:

ERRO[4887] Error while serving request: runtime error: invalid memory address or nil pointer dereference  request="&{DELETE /api/v1/policy HTTP/1.1 1 1 map[Origin:[http://localhost:27866] Content-Type:[application/yaml] Accept:[*/*] Content-Length:[188] Connection:[keep-alive] Authorization:[Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJuYW1lIjoiYWRtaW4iLCJleHAiOjE1Mjc5OTMzMzIsImlhdCI6MTUyNTQwMTMzMn0.cnRV8rz775DWk-0u7j5VrpjEpLcLgLcYgrfL3e7gJso] User-Agent:[Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.139 Safari/537.36] Referer:[http://localhost:27866/] Accept-Encoding:[gzip, deflate, br] Accept-Language:[en-US,en;q=0.9,ru;q=0.8] Cookie:[_ga=GA1.1.1658362583.1509052047; ATC-Authorization=\"Bearer eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJjc3JmIjoiYzYzMDY4MzMxMTA4Zjc5NTczZGViNjk2NTkxZDI4NjI1MzU3MTJiMWQzYWMwYTFjNWZmMjhiNDQ3NTZjNTIwMSIsImV4cCI6MTUyNzg4NzMxNCwiaXNBZG1pbiI6dHJ1ZSwidGVhbU5hbWUiOiJtYWluIn0.gof5lF0BwIn7nXP99b9uPtC1lL_zLITG2MzBeIR12Yp5hGz8niDUsYLb5EKD8K-EG6QJhtIWC88l3TE0_NsrJemgV7S1VJQoBNrRWbbotmZ8NrKjcFqy5p74G2yqTMc2IzEIRLLes5GtxwXu4oZ_1g6BMLA-Un77Pe7j2nYfSuu1nkcChBqtHHBWC8_u1hvPhdV1vhLzKK75XJxcqzbt12D6nuXP3nCGd6SrbSZP0tRVbqAR9rCUZi8DRgA4ODKMy3aXY2UscRo3lM60pRuotB8VYNahO270r7sjvgiuvLzIpbJtbvEUDdhHKYTlm4eNRx4b8uyqsM2lLvVBGez-kg\"]] 0xc425720a00 <nil> 188 [] false localhost:27866 map[] map[] <nil> map[] [::1]:52050 /api/v1/policy <nil> <nil> <nil> 0xc424978600}"
DEBU[4887] goroutine 21137 [running]:
runtime/debug.Stack(0xc400000005, 0x2cf6609, 0x1f)
	/usr/local/Cellar/go/1.10.2/libexec/src/runtime/debug/stack.go:24 +0xa7
github.com/Aptomi/aptomi/pkg/api/middleware.(*panicHandler).ServeHTTP.func1(0xc424db6600, 0xc420370960, 0x2e872a0, 0xc422546fc0)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/pkg/api/middleware/recovery.go:30 +0x26b
panic(0x2a45e80, 0x40b5bf0)
	/usr/local/Cellar/go/1.10.2/libexec/src/runtime/panic.go:502 +0x229
github.com/Aptomi/aptomi/pkg/engine.(*Revision).GetGeneration(...)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/pkg/engine/revision.go:74
github.com/Aptomi/aptomi/pkg/api.(*coreAPI).handlePolicyDelete(0xc4200b6ff0, 0x48818e0, 0xc424978510, 0xc424db6600, 0x0, 0x0, 0x0)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/pkg/api/policy.go:310 +0xaad
github.com/Aptomi/aptomi/pkg/api.(*coreAPI).(github.com/Aptomi/aptomi/pkg/api.handlePolicyDelete)-fm(0x48818e0, 0xc424978510, 0xc424db6600, 0x0, 0x0, 0x0)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/pkg/api/api.go:57 +0x66
github.com/Aptomi/aptomi/pkg/api.(*coreAPI).auth.func1(0x48818e0, 0xc424978510, 0xc424db6600, 0x0, 0x0, 0x0)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/pkg/api/auth.go:100 +0x209
github.com/Aptomi/aptomi/vendor/github.com/julienschmidt/httprouter.(*Router).ServeHTTP(0xc42095db90, 0x48818e0, 0xc424978510, 0xc424db6600)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/vendor/github.com/julienschmidt/httprouter/router.go:299 +0x6c1
github.com/Aptomi/aptomi/vendor/github.com/gorilla/handlers.combinedLoggingHandler.ServeHTTP(0x2e6b400, 0xc4200b4008, 0x2e657e0, 0xc42095db90, 0x2e872a0, 0xc422546fc0, 0xc424db6600)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/vendor/github.com/gorilla/handlers/handlers.go:77 +0x123
github.com/Aptomi/aptomi/pkg/api/middleware.(*panicHandler).ServeHTTP(0xc420370960, 0x2e872a0, 0xc422546fc0, 0xc424db6600)
	/Users/ralekseenkov-mini/go/src/github.com/Aptomi/aptomi/pkg/api/middleware/recovery.go:39 +0x99
net/http.serverHandler.ServeHTTP(0xc420d5bc70, 0x2e872a0, 0xc422546fc0, 0xc424db6600)
	/usr/local/Cellar/go/1.10.2/libexec/src/net/http/server.go:2694 +0xbc
net/http.(*conn).serve(0xc420e281e0, 0x2e8a860, 0xc4237ba000)
	/usr/local/Cellar/go/1.10.2/libexec/src/net/http/server.go:1830 +0x651
created by net/http.(*Server).Serve
	/usr/local/Cellar/go/1.10.2/libexec/src/net/http/server.go:2795 +0x27b