add a debug-recrawl command and some tests

Summary:
Add a way to test recrawls, which were fixed up in D922987. Add tests for query,
subscribe and trigger.

Test Plan:
`arc unit --everything` both with and without `WATCHMAN_VALGRIND=1` with the
added tests.

Reviewers: wez

Differential Revision: https://phabricator.fb.com/D922990

Task ID: 2718659

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extra : rebase_source : a09646a5253739e6acac787493ce8b1505f1dc94
extra : amend_source : df53f2ed8eee59d509b6936c198f1d098fd8d6ed