mongodb transport - evaluate connection
Merge pull request #363 from buckensl/master

MongoDB transport: Channel constructor doesn't evaluate the connection
Optional TTL support for MongoDB transport. AMQP TTL headers: x-message-ttl and x-expires
are used to produce MongoDB's TTL indexes.
Rearrange methods at MongoDB channel class
Another MongoDB transport clean up and refactor. Use of transport
options facility to get connection defaults and collection names.