Add LCB_IOPS_NAME and LCB_IOPS_SYMBOL (in addition to the older names)

We previously had LIBCOUCHBASE_EVENT_PLUGIN_NAME=.. -- let's use
something starting with LCB, and shorter.

Change-Id: Id4d52e2ee5956836fcef4b6a624357ffb0ee0877
Reviewed-on: http://review.couchbase.org/38776
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>