Provide LCB_INTERNAL_API alternative to gethrtime()

This allows pillowfight to be compiled without linking against the
library's internal source code.

Change-Id: I4f8aa1912754598d281a57ba99a5260cc70d2c60
Reviewed-on: http://review.couchbase.org/43539
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>