lcb_cntl: add helpers for uint32

This allows one-liners to be used for things like timeouts; thus e.g.

lcb_cntl_setu32(instance, LCB_CNTL_OP_TIMEOUT, 2500000)

Change-Id: I7531529b011b2055b529136576db5b26163468d4
Reviewed-on: http://review.couchbase.org/36142
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>