Fixes for Cassandra C++-driver

I made two fixes:
1. Fix preventing possible data corruption in cql_cluster_impl when several threads change _connected_sessions - map.

2. Sometimes I receive CQL_OPCODE_ERROR result in method cql:cql_connection_impl_t::body_read_handle after call of  cql:cql_connection_impl_t::options_write. 
This causes holding of  the caller in cql::cql_session_impl_t::allocate_connection for  waiting for shared_future forever. The proposed workaround prevents it.