Added support for MEMCACHED_BEHAVIOR_IO_KEY_PREFETCH

Added support for MEMCACHED_BEHAVIOR_IO_KEY_PREFETCH

From the man page:
The binary protocol works a bit different than the textual protocol in the
way that a multiget is implemented as a pipe of single get-operations that
is sent to the server in a chunk. If you use large multigets you may improve
the latency by setting this value so that you send out the first chunk of
requests when you hit the specified limit, allowing the server to start
processing the request and send the data back while the rest of the
requests is created.