Bumped mqtt and mows dependency.
Bumped v0.21.6. Update HISTORY.
add db to redis backend persistence options
merge from branch fix_150
Merge pull request #152 from mocheng/fix_150

Fix #150: add db to redis backend persistence options
correct usage of redis LRANGE command
Merge pull request #154 from mocheng/fix_151

Fix #151
Bumped v0.21.7.