Change transaction semantic and improve support

These changes are mainly due to the fact they ServiceMix now supports multiple flows.
A few changes are needed:

• if an exchange is created and sent transactionally, the transaction will flow with the exchange if it is sent using the sendSync method.
  If it is sent using the send method (asynchronous), the flow should be the transaction boundary.
• the StFlow can not support transactional exchanges due to possible deadlocks
• once an exchange has been sent using a flow, the same flow will be used every time this exchange is sent