[BUG] binary return data causes tracebacks in many returners

Description
Some returners return binary data. The default returner uses msgpack, which can handle binary data.

19/40 returners in salt/returners use salt.utils.json.dumps, which cannot handle binary data.

Maybe binary data should be b64 encoded instead of the return data being dropped?

grep 'json.dumps' salt/returners | awk -F : '{print $1}' | uniq:

cassandra_cql_return.py
couchbase_return.py
couchdb_return.py
elasticsearch_return.py
etcd_return.py
highstate_return.py
influxdb_return.py
kafka_return.py
mattermost_returner.py
memcache_return.py
mysql.py
odbc.py
postgres.py
postgres_local_cache.py
redis_return.py
slack_webhook_return.py
splunk.py
sqlite3_return.py
syslog_return.py

Start in salt/returners and inspect the listed returner files found by the json.dumps grep. Reproduce a return containing binary data, compare the affected returners with the default msgpack returner, and clarify a consistent handling policy. Done means the affected returners no longer traceback or silently drop binary return data.