The fix: when finished with a FunctionDefinition, only flush when the
number of usages reaches the chunk size. This should prevent a lot of
chunks with a low number of usages for files with short methods.

The tuning: for files larger than 10000 lines, use a larger chunk size
to prevent the UI thread from having to re-layout/re-paint too often.

Change-Id: I419174d306b8380c6fa8402825767e26c73f62ec
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>