memcache_stats: write a row for each server even if we can't get stats
memcache_stats: write a row for each server even if we can't get stats

Refactored stats fetching function a bit to always write a line for each
defined server even if stat fetching fails.  Also changed the format a bit
with an empty line after each server instead of just one empty line at the
beginning of the stat output.

ohmu/pgmemcache,ohmu/pgmemcache,saaros/pgmemcache,ohmu/pgmemcache