New Hook rc/watchlist hook ChangesListInitRows

An extension hooking into recent changes may need to load additional
data that did not fit into the recentchanges table.  This hook gives
extensions an opportunity to see the full result prior to rendering
and batch load where approprite rather than loading piecemeal during
the render process.

This is implemented as an optional method called by the ChangesList
consumer, since the ChangesList never sees the full result set. Hook
implementers must be able to work regardless of the hook being called,
they just have the oportunity to be more efficient when it is called.

Change-Id: If74ae600ffba949364dd381dd3d466cbbaa27286