Refactor some code related to the transaction manager

[jianglai]

Removed the deprecation mark as it is natural to expose methods related
to a transaction like getting the entity manager or checking if one is
in a transaction through the transaction manager interface.

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This change is 

[jianglai]

Reviewed 5 of 5 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @weiminyu)

[weiminyu]

The changes rolled back was part of the attempt to fix the misleading use of tm() and replicatedTm() methods,
which appears to control which DB is in use but in fact is not in control.

The attempted fix was to pass a handle to JPA methods to the transaction body. It is
infeasible, with the biggest issue being Java cannot deduce the type of a lambda with parameters.
Everywhere a transaction result is not assigned to a variable first, the code needs to be rewritten.

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)

[weiminyu]

Reviewed 1 of 5 files at r1, 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)

[jianglai]

Yeah, I think we probably gave up on that idea. Still, the question remains that calling replicatTm() doesn't always do what you want if you call it within a transaction...

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)