Connectors should be able to enroll in existing transactions

Read/write connectors (e.g. the JCR connector) need to be able to enroll in an existing transaction, from a repository perspective and need to be able to work "transactionally" with document changes.

The easiest way to do that, is to expose the repository's TransactionManager to the connector, so that the connector can enroll itself (or another resource) as an XAResource.