A new UVE message is added in collector to keep track of the sandesh

messages statistics per minute. Current counters track only the running
sum since epoch.
Code changes:
A new UVE sandesh message is added in collector_uve.sandesh
In generator.cc, functions to update the statistics counter for
each messages and get the per minute statistics for each mesasge
is added.
In collector.cc, a function that gets called every min to get all
the stats collected from each generator is provided. After that it
gets sent.