pybind: Some fixes on semantics

About signals like SIGINT, I think it is up to the library client (e.g. ceph.in) to handle these scenarios and create threads on calls. Creating threads to handle signals on python side seems not to be responsibility of a wrapper library. 
Also, some use cases may not need this at all (non-console clients can simply gracefully finish the request after timeouts set by configs). Some environments may even not allow threads.