msg: reset ProtocolV2's frame assembler in appropriate thread

msg: reset ProtocolV2's frame assembler in appropriate thread

`set_is_rev1()` actually resets the internal state of `FrameAssembler`,
so -- to avoid racing -- it should be performed by the same thread that
takes care about executing `ProtocolV2::write_message()`.

```cppp
void set_is_rev1(bool is_rev1) {
  m_descs.clear();
  m_flags = 0;
  m_is_rev1 = is_rev1;
}
```

See comments in the tracker for analysis.

Fixes: https://tracker.ceph.com/issues/55851
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>