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box: enable synchro replication for system spaces when limbo is claimed

Mixing asynchronous and synchronous replication can lead to a split-brain
state, when the new limbo owner doesn't have all the asynchronous
transactions of the former owner. By default, the system spaces remain
asynchronous even when all the user-defined spaces are synchronous.

To fix this, let's enable synchronous replication for system spaces when
the synchronous queue is claimed.

We need to do it early on in `txn_limbo_req_prepare` rather than in
`txn_limbo_read_promote` , because the latter is called only after the
`IPROTO_PROMOTE` WAL write succeeds. While the `IPROTO_PROMOTE` WAL write
is in progress, the user will be able to perform some asynchronous
operations on system spaces, and these operations will be written to the
journal after the `IPROTO_PROMOTE` WAL write. This will lead to
asynchronous operations on system spaces inside the “synchronous” part of
the journal. Taking action in `txn_limbo_req_prepare` will ensure that no
changes to systems spaces can be made during limbo ownership acquisition.

There can be 4 cases for limbo ownership change, the actions are taken on
all instances of the replicaset:
|---------------|---------------|------------------------------|
|   Old owner   |   New owner   |           Action             |
|---------------|---------------|------------------------------|
|      nil      |      nil      |          no action           |
|---------------|---------------|------------------------------|
|      nil      | some instance |  enable synchro replication  |
|---------------|---------------|------------------------------|
| some instance |      nil      |  disable synchro replication |
|---------------|---------------|------------------------------|
| some instance | some instance |          no action           |
|---------------|---------------|------------------------------|

We do not enable synchronous replication for the following sy... (continued)

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