core/state/snapshot: simplify snapshot rebuild

[ARR4N]

This PR is purely for improved readability; I was doing work involving the file and think this may help others who are trying to understand what's going on.

1. snapshot.Tree.Rebuild() now returns a function that blocks until regeneration is complete, allowing Tree.waitBuild() to be removed entirely as all it did was search for the done channel behind this new function.
2. Its usage inside New() is also simplified by (a) only waiting if !AsyncBuild; and (b) avoiding the double negative of if !NoBuild.

[holiman]

Very elegant, looks good to me!

[holiman]

Seems to get stuck though :)

goroutine 22 [runnable]:
github.com/ethereum/go-ethereum/core/state/snapshot.New({0xc0003be008?, 0x2?, 0x0?, 0x0?}, {0x7f52d24be8f8, 0xc00003a680}, 0xc00045e030, {0xec, 0xec, 0xdf, ...})
	/home/appveyor/projects/go-ethereum/core/state/snapshot/snapshot.go:222 +0x2b2
github.com/ethereum/go-ethereum/core.NewBlockChain({0xac60f0, 0xc00003a680}, 0x0?, 0xc00003fe48, 0x0, {0xac3cb0, 0xc0001006c0}, {0x0, 0x0, 0x0, ...}, ...)
	/home/appveyor/projects/go-ethereum/core/blockchain.go:449 +0x182a
github.com/ethereum/go-ethereum/consensus/clique.TestReimportMirroredState(0xc00015b520)

[ARR4N]

Seems to get stuck though :)

Whoops, I read the for head != nil as if head != nil. Thanks for reverting that.

[ARR4N]

@holiman sorry for the post-approval push. I just made the for loop obvious as I and another developer I spoke to both assumed it was an if statement.