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limbo: introduce explicit state

The txn_limbo::state now is one member which clearly reflects the
state of the limbo: inactive, leader, or replica.

Previously these states also existed but had to be deducted from
other properties.

With the new approach it became possible to remove some of the
other state-flags, simplify others, and simplify the limbo's API
in general.

For example, there is no more such states as "fenced" or "frozen".
Because these were very high-level descriptions of events like
when a leader steps down due to quorum loss or a new term
appearing or when the instance was just restarted. These were
events, not states. The state in these cases is the limbo becoming
a "replica" instead of "leader".

With such approach the concept of "unfencing" also disappears.
Because a replica can't just become a leader even if the quorum
is restored. It anyway needs to be elected again.

The most important parts of this commit are the state itself
and the new flag is_transition_in_progress. The new flag
previously didn't exist explicitly. These new things will be
needed in scope of #8095, where ownership transition will actually
be in progress for quite a while, not only during PROMOTE journal
write.

Part of #8095

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=tested

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