auth: add rwlock to AuthClientHandler to prevent races

For cephx, build_authorizer reads a bunch of state (especially the
current session_key) which can be updated by the MonClient. With no
locks held, Pipe::connect() calls SimpleMessenger::get_authorizer()
which ends up calling RadosClient::get_authorizer() and then
AuthClientHandler::bulid_authorizer(). This unsafe usage can lead to
crashes like:

Program terminated with signal 11, Segmentation fault.
370 common/buffer.cc: No such file or directory.
in common/buffer.cc
(gdb) bt

and

Error in `qemu-system-x86_64': invalid fastbin entry (free): 0x00007ff12887ff20
*** ======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x80a46)[0x7ff3dea1fa46]
/usr/lib/librados.so.2(+0x29eb03)[0x7ff3e3d43b03]
/usr/lib/librados.so.2(_ZNK9CryptoKey7encryptEP11CephContextRKN4ceph6buffer4listERS4_RSs+0x71)[0x7ff3e3d42661]
/usr/lib/librados.so.2(_Z21encode_encrypt_enc_blIN4ceph6buffer4listEEvP11CephContextRKT_RK9CryptoKeyRS2_RSs+0xfe)[0x7ff3e3d417de]
/usr/lib/librados.so.2(_Z14encode_encryptIN4ceph6buffer4listEEiP11CephContextRKT_RK9CryptoKeyRS2_RSs+0xa2)[0x7ff3e3d41912]
/usr/lib/librados.so.2(_ZN19CephxSessionHandler12sign_messageEP7Message+0x242)[0x7ff3e3d40de2]
/usr/lib/librados.so.2(_ZN4Pipe6writerEv+0x92b)[0x7ff3e3e61b2b]
/usr/lib/librados.so.2(_ZN4Pipe6Writer5entryEv+0xd)[0x7ff3e3e6c7fd]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x7f8e)[0x7ff3ded6ff8e]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7ff3dea99a0d]

Fix this by adding an rwlock to AuthClientHandler. A simpler fix would
be to move RadosClient::get_authorizer() into the MonClient() under
the MonClient lock, but this would not catch all uses of other
Authorizer, e.g. for verify_authorizer() and it would serialize
independent connection attempts.

This mainly matters for cephx, but none and unknown can have the
global_id reset as well.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>