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limbo: encapsulate limbo promotion logic

Promotion logic of the limbo was scattered across the following
places quite messy:
1. box/raft which must invoke box_promote_qsync().
2. box/box's box_promote() had to check terms, collect acks,
  trigger the low-level promotion request.
3. box/txn_limbo's txn_limbo_req_promote() did the actual final
  write + commit.

While the code is still imperfect, it is quite clear, that box.cc
must not be digging so deeply into internals of the limbo.

This whole logic now is moved into the limbo's internal code and
exposed as one simple function call. Which does one thing - makes
sure the limbo ownership is caught up with Raft's state machine.

This in turn also allows to entirely hide the selection of
promotion LSN. It will be necessary in the future, when
synchronous PROMOTE will be implemented, and its commit LSN logic
will be relatively complicated and very internal.

As a bonus alongside the code just got quite simpler.

Part of #8095

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

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