separate w_root_new and w_root_delref into init and teardown steps

Summary:
Upcoming patches will require us to pave over a part of the state for recrawls.
Separate initialization and destruction into parts that we will initialize at recrawl,
and parts that we won't. Use a boolean variable called `initialized` as a sentinel beyond
which everything is paved over.

Test Plan: `arc unit --everything`. Will be tested for real in upcoming patches.

Reviewers: wez

Differential Revision: https://phabricator.fb.com/D922983

Task ID: 2718659

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extra : rebase_source : 77245f546a379772abe0d4fe0cbf8808bea4dfb4