fix 148
Merge pull request #156 from mocheng/fix_148

fix 148
Fix in level persistence to allow more than 1 offline msg per second

Bumped level-ttl and memdown dependencies.
Bumped v0.21.8. Updated HISTORY.md.
Really fixed LevelUp concurrent storePacket issue.

Closes #160.
Fixed spurious MongoDB failures due to bad asynchronicity.
Improved reliability of LevelUp persistence tests.

Still the 'database not open spurious failure remains'.
Bumped v0.21.9.
correct deleteOfflinePacket param name
Merge pull request #163 from mocheng/param-name

correct deleteOfflinePacket param name
use pexpire for redis persistence
report error as connected redis instance is < 2.6
Merge pull request #164 from mocheng/fix_158_redis

fix 158: add packets ttl implementation on redis persistence
emit emit unsubscribed for server even there is no subscrition yet
Merge pull request #167 from mocheng/fix_166

emit emit unsubscribed for server even there is no subscrition yet
add opts.forwardFilter
Removed topic rewriting.

Topics are no longer normalised from e.g a///topic to a/topic. This
matches the behaviour as clarified by the Oasis MQTT spec. This will
lead to unexpected behaviour if you were using topics of this form.
cit. @ralight from Mosquitto changelog.
add server.authorizeForward method
Merge pull request #168 from mocheng/filter_forward

added `authorizeForward`.
Merge branch 'master' of github.com:mcollina/mosca