added retry for "execute" queries
messages as shared_ptrs to defeat segfaults
cql_session_impl_t with enable_shared_from_this
cql_connection_impl_t with enable_shared_from_this (thanks mhfrantz)
removed raw pointers to connections + cql_control_connection with enable_shared_from_this
io_service passed by shared_ptr everywhere
passing messages as shared_ptrs