delta operations: fix negative offsets in increment and decrement

Memcached defines increment and decrement functions with uint64_t offsets.
PostgreSQL only has signed types, so we had some code that was supposed to
throw an error if a negative offset was passed in.  But the variable it was
looking at was unsigned, so the comparison was never true.

Instead of just fixing it to use a signed operation, let's invert the
operation if negative offset is passed in, effectively turning increment(-1)
into decrement(1).
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.