[plume-db & plume-db-querydsl] Provide interfaces for TransactionManager

[amanteaux]

This will enable to provide an easier way to override the TransactionManager.

The current TransactionManager will be called HikariTransactionManager, whereas the interface will still provide default implementation of executeAndReturn and execute based on the implementation of dataSource.

This will impact TransactionManagerQuerydsl:

• It will implement TransactionManager instead of extending it
• It will contain an instance of TransactionManager to delegate the corresponding methods

[amanteaux]

Implemented in #35 where the TransactionManager now depends only on the DataSource