txn_limbo: do not confirm/rollback anything after restart

It's important for the synchro queue owner to not finalize any of the
pending synchronous transactions after restart.

Since the node was down for some time the chances are pretty high it was
deposed by some new leader during its downtime. It means that the node
might not know yet that it's transactions were already finalized by someone
else.

So, any arbitrary finalization might lead to a future split-brain, once the
remote PROMOTE finally reaches the local node.

Let's fix this by adding a new reason for the limbo to be frozen - a
queue owner has recovered but has not issued a new PROMOTE locally and
hasn't received any PROMOTE requests from the remote nodes.

Once the first PROMOTE is issued or received, it's safe to return to the
old mode of operation.

So, now the synchro queue owner starts in "frozen" state and can't
CONFIRM, ROLLBACK or issue new transactions until either issuing a
PROMOTE or receiving a PROMOTE from some remote node.

This also required modifying box.ctl.promote() behaviour: it's no
longer a no-op on a synchro queue owner, when elections are disabled and
the queue is frozen due to restart.

Also fix the tests, which assumed the queue owner is writeable after a
restart. gh-5298 test was partially deleted, because it became pointless.

And while we are at it, remove the double run of gh-5288 test. It is
storage engine agnostic, so there's no point in running it for both
memtx and vinyl.

Part-of #5295

NO_CHANGELOG=covered by previous commit

@TarantoolBot document
Title: ER_READONLY error receives new reasons

When box.info.ro_reason is "synchro" and some operation throws an
ER_READONLY error, this error now might include the following reason:
```
Can't modify data on a read-only instance - synchro queue with term 2
belongs to 1 (06c05d18-456e-4db3-ac4c-b8d0f291fd92) and is frozen due to
fencing
```
This means that the current instance is indeed the synchro queue owner,
but it has noticed, that someone else in the cluster might start new
elections or might overtake the synchro queue soon.
This may be also detected by `box.info.election.term` becoming greater than
`box.info.synchro.queue.term` (this is the case for the second error
message).
There is also a slightly different error message:
```
Can't modify data on a read-only instance - synchro queue with term 2
belongs to 1 (06c05d18-456e-4db3-ac4c-b8d0f291fd92) and is frozen until
promotion
```
This means that the node simply cannot guarantee that it is still the
synchro queue owner (for example, after a restart, when a node still thinks
it is the queue owner, but someone else in the cluster has already
overtaken the queue).