Summary: `fastverifier` was sometimes being overriden by `shallowverifier` when remotefilelog was enabled. Since the latter is a subset of the former, let's just fold both into the core verifier code backed by a config, `verify.skipmanifests`, that we can default to true.
Reviewed By: DurhamG
Differential Revision: D6882222
fbshipit-source-id: 9f337ca031a070425ccdc9ee02f6765e68436da9
