Add a Post Commit Hook for Jooq Transactions

[johnlcox]

In order to do things like caching of query/insertion results deep within a chain of nested transactions, it would be helpful to have some sort of callback mechanic that would trigger when the root/top-level transaction is committed.

Some possible classes that could be created to help facilitate this:

• TransactionCommitHooks - Could have a register method that nested transactional code could register lambda expressions with to be executed when the top-level commit is completed. Probably would use ThreadLocal for this since ThreadLocal is already used for DslContextProviderImpl.
• Enhance DslContextProviderImpl to call the proper set of registered hooks when the unit of work is ended.

Should the hooks fire for rollback? Maybe the lambda takes a bool parameter that says whether it was successful or not? Alternatively, maybe there is a separate success/failure callback? What about nested savepoint commit callbacks? Is there value to having callbacks for all the different levels and some sort of indicator whether it's nested or root?

References: django uses a callback function to handle performing tasks after commit, https://docs.djangoproject.com/en/2.1/topics/db/transactions/#performing-actions-after-commit

[johnlcox]

As I've thought about this more I'm less sure one what this class should be named. The javadoc on it must make clear that it's tied to the @JooqTransactional ThreadLocal transaction management.

Answers to some of the questions:

• Should the hooks fire for rollback? Not at this time, but it should be designed in such a way that it can be enhanced, passively, to support rollback hooks.
• Maybe the lambda takes a bool parameter that says whether it was successful or not? I don't think so. It should instead be a method that only gets fired on successful commit.
• maybe there is a separate success/failure callback? Just success for now.
• What about nested savepoint commit callbacks? The callbacks should get called in the order they are registered. If a savepoint gets rolled back, callbacks part of that savepoint should not fire, but any callbacks that are part of a parent savepoint that is successful should fire.
• Is there value to having callbacks for all the different levels and some sort of indicator whether it's nested or root? Yes, there is value, but they should just be called in the order they are registered and only fire when the top level commit is successful.

[johnlcox]

Fixed by #140