objectcache: optimize MemcachedPeclBagOStuff::*Multi() write methods

objectcache: optimize MemcachedPeclBagOStuff::*Multi() write methods

Make use of Memcached::OPT_NO_BLOCK and Memcached::OPT_BUFFER_WRITES
at selective points to avoid slow internal blocking for loops in the
PECL extension methods. As this mode makes the return values less
meaningful, this is only triggered only if WRITE_BACKGROUND is
provided (it only catches some glaring format or size errors that the
client runs into).

Leave changeTTLMulti() to the default as the above approach would not
make it faster, since the diver does not buffer for that command.

Expand mctest.php to include *Multi() methods and also break down the
single key method timings by method.

Change-Id: I11fc58e1a2296778b2526a0b2f3103fef1baba0c