[Java Client AMQP 0-x] Connection#start is not thread-safe

The JMS spec says that Connections may be used from several threads concurrently.
However, our implementation of Connection#start is currently not thread-safe. It iterates over its list of sessions and calls Session#start. The session rightfully (this is specified in the JMS spec as well) assumes that it is being called from a single thread.

An example of the impact of this is that a session can be used (by Connection#start) after it has been closed (via Session#close) leading to an Exception due to the broker receiving a frame with an unknown channelId.

Note: The comment on Connection#start stating that JMS allows for non thread-safe implementations seems to be in error!