timeval: Coalesce backtraces with counts.

timeval: Coalesce backtraces with counts.

With this patch, `ovs-appctl backtrace` will return a unique list
of backtraces and a count of how many times it has been recorded.
This work had previously been done by ovs-parse-backtrace. However,
in future patches poll-loop will begin logging backtraces as a
matter of course.  At this point, coalescing the backtraces will
help keep these log messages brief.

Signed-off-by: Ethan Jackson <ethan@nicira.com>