Pop _unique_id when checking for duplicates

Drivers should not be returning messages with any driver-specific
wire format fields included. Right now, the rabbit and qpid drivers are
leaking the _unique_id field.

Avoid this simply by popping _unique_id when we're checking for dups.

Change-Id: Icbfb02ee66aebe5ef97a9a2502d8a0745e07bd8b
Merge "Pop _unique_id when checking for duplicates"