[libcommhistory] Resolve event ContactGroup startTime to most recent

A ContactGroup (conversation) with multiple group objects uses startTime
timestamp from group object with OLDEST startTime.
Fix to use most recent startTime.
Otherwise startTime does not match to latestMessage and leads to wrong
conversation (group) startTime on UI.
Merge pull request #97 from tkenakka/jb24278-msgs-group-timestamp-broken

[libcommhistory] Resolve event ContactGroup startTime to most recent
Bump version to 1.7.21
[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
Merge pull request #99 from jpetrell/calllog-limit

Add temporary limit property to call proxy model