transactions: Support transaction coordinator leadership transfer

[bharathv]

Cover letter

The patch supports graceful leadership transfer scenario for transaction coordinator (tm_stm). High level approach is as follows.

• Adds a transferring flag to tm_transaction state. Defaults to false but is true for transfer-in-progress txns.
• Upon receiving a leadership transfer request, flip this flag and flush ongoing txns to log
• New leader applies them in-memory and lazily resets the flag upon access but before updating their state.
• If the flag is set and the term does not match, new leader expires them so that we don't read any dirty uncommitted txns.

A side effect of this refactoring is that the transactions can now appear in log/mem state, so the patch refactors to lookup in both places wherever needed.

Fixes #6214

Backport Required

• [x] not a bug fix
• [ ] issue does not exist in previous branches
• [ ] papercut/not impactful enough to backport
• [x] v22.2.x
• [ ] v22.1.x
• [ ] v21.11.x

UX changes

• none

Release notes

• none

Improvements

• Transactions can now span leadership changes of transaction coordinator.

[VadimPlh]

Put * none for UX changes and Release notes

[VadimPlh]

Do we need to add some chaos test for it?

[bharathv]

/ci-repeat 5

[bharathv]

Test failure #6601