Make `DbCleaner` and `BeforeAllAdapter` to support many connections

What does this MR do?

This generalises support for many databases based on all open connection pools. This fixes support for handling many transactions across many connection for many databases.

Related to:

• #336896 (closed)

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.