omcache use: set timeouts for commands instead of never waiting for writes
omcache use: set timeouts for commands instead of never waiting for writes

Previously all omcache calls using the libmemcached wrapper api (most of the
write commands) always had a timeout of `0` which meant that OMcache didn't
wait for results and just returned `MEMCACHED_BUFFERED` immediately.  That's
fine for some workloads but not ok for others, including the regression
tests.

Add a hardcoded non-zero timeout for writes for now, we should make this
runtime configurable at some point.  Requires currently unreleased OMcache.

ohmu/pgmemcache,ohmu/pgmemcache,ohmu/pgmemcache