Don't share connection pool between driver object

Each driver instance must use it's own connection pool.

This removes the last global state of qpid and rabbitmq driver
Make the relation between classes more simple.

The previous behavior was not very safe, as explained in the bug report.

And also, this is a first step to replace this custom connection pool
handling by the kombu one.

Closes bug: #1397925
Partial bug: #1397339

Change-Id: Iecd2b39c76417d9ac081d46810f72eb6e38edfda