Issue 24900 part 1: document and refactor, part 2: add first step in PoolManagerImpl unit test

[escay]

Draft for now, I would like to add more unit tests and I would like to remove a number of TODO statements I added in ConnectionPool and PoolManagerImpl classes.

Part 1 for issue #24900 Document and rename some ResourceHandle fields. Document and rename some ConnectionPool fields. Remove some unused methods and constructor parameters. Functional / possible incompatibility changes in DataStructureFactory and in ResourceHandle due to constructor changes.
(Maybe part 1 is a bit too much, I altered a few interfaces and removed unused parameters)

Part 2 for issue #24900 Start with a new PoolManagertImpl unit test to understand enlisted versus busy states of Resource handles and the wiring inside a transaction to keep track of all used resources.

[escay]

Part 3 for issue #24900 adds some more documentation. Main item of this commit is:

• Changed unit test to show 2 points where I would expect different behaviour. In case of resourceAbortOccurred and resourceErrorOccurred the connectionErrorOccurred flag of the resource is not set, while it is set in badConnectionClosed logic.
• Changed unit test to show removing resources from a transaction seems to behave as expected for basic scenarios.