BE-260 looking to address two issues:

cql::cql_session_impl_t::cql_connections_collection_t*
cql::cql_session_impl_t::add_to_connection_pool

passes out an unlocked/unprotected pointer to cql_connections_collection_t in
cql_session_impl_t::connect

adding in a mutex above that, since this pointer could become invalidated by
activity in other threads.

Also, callbacks on bad connections in cql_session_impl_t::allocate_connection
were resulting in many valgrind errors. Holding on to these in a dump list.
That will introduce some data leakage, but can come back to make it clear out
say based on some time interval. These valgrind error look to have been associated
with segfaults.

Note that all of these issues surfaced in an env where the cassandra servers were
going up and down. That is where these issues came to light for us.