Fix for bug 969996

Delay chunk rendering by grouping chunks into larger
sets before calling renderChunk.  First set size will
be the same as the current FIRST_CHUNK_SIZE (6).  Subsequent
sets will cap at INCREMENTAL_RENDER_SIZE (60).

The goal is to reduce reflows on rendering the call log.

Also fixed the following bugs:
  _groupCounter never gets incremented if there's only one log entry per day
  Change currDate and prevDate comparison to more readable form

Added unit tests for call log rendering of various sizes.
Merge pull request #16107 from yor-mozilla-com/fix969996

Fix for bug 969996 r=rik