vinyl: abort rw transactions when instance switches to ro

A Vinyl transaction may yield while having a non-empty write set. This
opens a time window for the instance to switch to read-only mode. Since
we check ro flag only before executing a DML request, the transaction
would successfully commit in such a case, breaking the assumption that
no writes are possible on an instance after box.cfg{read_only=true}
returns. In particular, this breaks master-replica switching logic.

Fix this by aborting all local rw transactions before switching to
read-only mode. Note, remote rw transactions must not be aborted,
because they ignore ro flag.

Closes #4016