txn_limbo: rework limbo->frozen flag

Soon there will be more reasons for a transaction limbo to be frozen.
Let's make the limbo->frozen flag a bitmap and rename it to
limno->frozen_reasons.
The first bit, named frozen_due_to_fencing, represents the only current
reason for the limbo to be frozen.
While we are at it, rename txn_limbo_(un)freeze to txn_limbo_(un)fence
to better reflect the situation.

Part-of #5295

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring