Provide an isDatatypeSupported API for engine

Change-Id: I2636ea5309347b538b23338d527d7b2cf38ccf13
Reviewed-on: http://review.couchbase.org/36737
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>