switch triggers to using clockspecs

Summary:
Currently triggers initialize `w_query_since` objects directly. Unfortunately
that won't work if the root number changes on recrawl, as is planned. Switch to using
`w_clockspec` objects. The dynamic creation makes me sad though.

Test Plan: `arc unit --everything` and also with `WATCHMAN_VALGRIND=1`

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D922912

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extra : rebase_source : 54559f876268c5576a02a163d3f8ddce763e1655
extra : amend_source : cd520234b314e9ad92583eb46d710f1e759fe4fb