reinitialize most state when recrawling

Summary:
We've found that recrawling doesn't currently clean up enough state.
Reinitialize most of the internal state of the root.

Test Plan:
`arc unit --everything` and the full Mercurial test suite with `-j16` -- real
tests will come in upcoming patches though.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D922987

Task ID: 2718659

--HG--
extra : rebase_source : 11fd3539f272b9db18627da28901f566f56e02a4
extra : amend_source : ae9a6a80d4f6d5be5c6c46843841f22d0743b63e