Summary:
This diff adds a new SearchIndexer class that each of the concrete
indexer implementations register with. This new class uses the
`isSearchIndexed` field in searchable classes to split indexing work
into chunks. It then times how long it takes to index each chunk and
schedules more work based on a target CPU percent. For example, if it
takes 150 ms to index the last chunk of work and the target CPU fraction
is 0.05 (i.e. 5% CPU use) then it will schedule the next increment of
indexing 3 seconds in the future.
Test Plan: Run locally, verify that indexing occurs in increments
Reviewers: juan, evan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D3707
