Fix goroutine leak by closing accounts manager on Node.Stop

[janos]

Fixes: #1142

While performing the test described in #1142 issue, a large number of gorutines were created and not terminated. Stack trace shows many goroutines:

goroutine 3370 [select]:
github.com/ethereum/go-ethereum/accounts.(*Manager).update(0xc0000b7520)
	/Users/janos/go/src/github.com/ethereum/go-ethereum/accounts/manager.go:95 +0x22a
created by github.com/ethereum/go-ethereum/accounts.NewManager
	/Users/janos/go/src/github.com/ethereum/go-ethereum/accounts/manager.go:68 +0x6f2

goroutine 4481 [select]:
github.com/ethereum/go-ethereum/accounts/keystore.(*watcher).loop(0xc0035ac9a0)
	/Users/janos/go/src/github.com/ethereum/go-ethereum/accounts/keystore/watch.go:94 +0x65b
created by github.com/ethereum/go-ethereum/accounts/keystore.(*watcher).start
	/Users/janos/go/src/github.com/ethereum/go-ethereum/accounts/keystore/watch.go:52 +0xb6

Accounts manager is created but not closed for node.Node, leaving large number of goroutines alive. This change closes accounts manager on Node.Stop, but Node initializes accounts manager in constructor, not no Start, so for Restart and Start to work as expected, accounts manager construction and closing is handled in Start as well, preserving the original required initialization in constructor.

Keystore watch creates fs watchers and we have noticed before a lot of (FSEvents.framework) FSEventStreamStart: register_with_server: ERROR: f2d_register_rpc() => (null) (-21) errors on macOS while running a large scale simulations. This change may improve simulations performance as it closes unneeded file watchers.

Note: Running tests described in #1142 time go test -race -count 100 github.com/ethereum/go-ethereum/swarm/network/simulation require changes from PR ethereum/go-ethereum#18464 to fix race conditions, but this PR solves goroutine leak. This tests run for about 20 minutes.

[frncmx]

lgtm 👍

[frncmx]

@janos I think in the upstream PR we should request a review from Felix and Peter, as I see they worked on this part mostly. That can take time, too.

So what do you think about submitting this upstream?

[janos]

@frncmx yes, I'll submit upstream, to speed things up.

[janos]

Submitted upstream ethereum/go-ethereum#18505.