Handle aquery build statements in a goroutine

Creation of build statements is largely parallelizable because each
action is independent apart from updates/reads to
depsetHashToArtifactPathsCache. Locally resulted in build statements
taking ~.45 seconds on staging mode to ~.02 seconds

Test: CI
Change-Id: Iab00c8394a9eab17353f71230885ff0870e17f24