[libcommhistory] Enable limiting for flat call model queries

It's easy to limit results for queries for call models which aren't
using tree mode, and limiting increases performance when the full
history isn't needed.
Merge pull request #98 from hmallat/flat-callmodel-limited-results

[libcommhistory] Enable limiting for flat model queries
Bunp version to 1.7.22
[libcommhistory] Add temporary limit property to call proxy model