set operations: return NULL on MEMCACHED_BUFFERED

Return NULL when a set-related function returns MEMCACHED_BUFFERED to
indicate that we don't really know the result of the action, but it wasn't
an error.
pgmemcache: flush client buffers on commit if configured so

Flush client buffers to memcache servers on commit if the new GUC
pgmemcache.flush_on_commit is enabled, otherwise some buffers may get
lost when the PostgreSQL backend terminates before everything has been
sent which is especially likely when the BUFFER_REQUESTS behavior is used.
Merge pull request #30 from saaros/buffer

Handle buffered requests