Added all filter tests.
Added checks for TTL when fetching info from infostore and from
groups, with associated unittests.
Added InfoStore.InfoCount() to find the count of infos amongst all
groups and within the non-group map. Augmented unittests.

Added InfoStore.BuildFilter(maxHops) which creates filters for peer
selection, limiting the keys to those for infos which are <=
a configurable number of hops. Added unittests.