scanner: Implement scan deadline timeout

Implemented a configurable timeout of 300s for
index scan operation. If the scan takes longer,
the scan coordinator will abort the scan and send
an error, ErrScanTimedOut to the client.

Change-Id: I4ae03cf3cab0f1defbc5636bdb8cf52d6f1a830c
Reviewed-on: http://review.couchbase.org/44077
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>