PYTHON-191 Add support for custom replication strategies.
Remove int type requirement, revert TypeError in favor of None return

also update tests
Don't use Exception.message in warn log (python3)
Correct meta logic for composite comparators, compact storage

PYTHON-192
Prevent NIE when unknown repl strategy with TA LBP

Unknown replication strategies would raise if token aware load balancing
used.
Also fixes a more general issue where TokenAware blows up with
strategies returning an empty mapping (i.e. LocalStrategy, and Unknown).
Improved var name in ReplicationStrategy

review input
Merge branch 'PYTHON-191'
Merge branch 'PYTHON-192'
Changelog update for interim tickets.