msg/async/ProtocolV1: fix locking around authorizer_buf

msg/async/ProtocolV1: fix locking around authorizer_buf

Fix two problems:

- we are accessing authorizer_buf without the connection lock, and
  under the lock we are modifying it (in connect()).
- if we receive two connect_msg's with a different length, we won't
  have a buffer that's large enough.

Fixes: http://tracker.ceph.com/issues/38524
Signed-off-by: Sage Weil <sage@redhat.com>