Log connecting client IP and port.
Ask p11 module to use OS locking.

Since p11proxy is a threaded application, the underlying p11
module should do locking (the proxy does no locking).
Add XXX comment about mutexes in init_args.
rpc_C_CloseAllSessions: Only close sessions opened by this thread.

Threads on pkcs11-proxy map to applications rather than threads in the
PKCS#11 (v2.2) specification. It is therefor important to not close
sessions opened by other threads in C_CloseAllSessions.