lcb_cntl_string(): allow timeout values to be epxressed as floats

Change-Id: Id60745a6266676e2eb7a0218318273e9f2246d18
Reviewed-on: http://review.couchbase.org/40986
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>