Store number of signatures count in memcached

Store the number of signatures for a petition in memcached if available,
to reduce number of expensive COUNT() queries performed.

Cache time in seconds can be set with $wgPetitionCountCacheTime
in LocalSettings.php, or will default to 24 hours.

Bug: 65851
Change-Id: I9e0b8263168cafeaa5b399dae217ff924f13eff1