cleanup a bunch of stuff related to compare and swap requests

Summary

• fix: used to sometimes report incorrect raft term immediately after restarting

• fix: use picodata's system connection pool for CAS requests

• fix: unify which term is used for cas predicates

CAS predicates should always contain the current raft term, because this is what's explicitly checked in proc_cas.

Note that this will sometimes result in a EntryTermMismatch error because the latest applied entry may have a different term in case the election has started but not finished yet.

We could add this check to all the client cas precondition checks, but it's not a big deal, because we handle the situation correctly anyways.

• refactor: move wait_index into compare_and_swap

• refactor: compare_and_swap now accepts a deadline instead of a timeout

• refactor: massage compare_and_swap a bit

• fix: unify all places where we call compare_and_swap and wait for the result

• fix: check if DdlPrepare was rolled back before waiting for DdlCommit

Rolled back as in raft log roll back, which can happen when a leader changes to one who doesn't have the entry we persisted. Not to be confused with DdlAbort.

• fix: cas requests are always reconstructed before retrying

• fix: TermMismatch was always retriable, when did it stop?

• doc: add comments to retriable error codes explaing why they are retriable

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• Cherry-pick to: none
• Docs follow-up: not necessary