Removed limit of 23 bytes for the client id.

Closes #82.
Removed two possible race conditions for offline messages.

The race conditions were:
1. the restoration of all subscriptions was done after connack, which
   means that there was a tiny window were a message could be lost.
2. the puback for a QoS 1 packet was sent before the write was
   concluded.
Bumped v0.15.0. Updated HISTORY.

Closes #73.