SPY-113: Implement underlying foundation for replica read.

This changeset implements the facilities that need to be in place
to handle replica reads up the stack (typically inside the
couchbase client library).

Change-Id: I7ba237ec559c14b6f1b9876b058fff583b56d020
Reviewed-on: http://review.couchbase.org/24749
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>