[tsm1] Change timestamp from time.Time to unixnano

[methane]

time.Time contains timezone loc *Location.
Since tsm1.Cache holds a lot of values in heap, there are many pointers in heap.
This makes GC heavy.

before:
$ go test -run=xxx -bench=BenchmarkCache
PASS
BenchmarkCacheFloatEntries-4         300       4019495 ns/op

after:
$ go test -run=xxx -bench=BenchmarkCache
PASS
BenchmarkCacheFloatEntries-4         500       3720449 ns/op

related issues:

• runtime: performance degradation for versions > 1.4.x golang/go#14189
• revert to go1.4.2 (and wait for go1.6+) #5217

---

• CHANGELOG.md updated
• Rebased/mergable
• Tests pass
• Sign CLA (if not already signed)

[jwilder]

Thanks @methane. I see similar improvements locally. There are other places where time.Time is used and a change like this might help them as well.

influxdb:influxdb jason$ go test -run non -bench BenchmarkCacheFloatEntries -benchmem ./tsdb/engine/tsm1/
PASS
BenchmarkCacheFloatEntries       500       3368979 ns/op      593239 B/op      30016 allocs/op

influxdb:influxdb jason$ go test -run non -bench BenchmarkCacheFloatEntries -benchmem ./tsdb/engine/tsm1/
PASS
BenchmarkCacheFloatEntries       500       3120040 ns/op      433206 B/op      30015 allocs/op

[methane]

I've seen profiler output posted on golang/go#14189.
I found most of memory is consumed by this cache.
So this is the largest part of memory consumption and GC time.

[e-dard]

LGTM 👍

[methane]

@toddboom Would you run the benchmark same to golang/go#14189 ?