Worker registers itself after a successful reconnect

We noticed that the current gearman-coffee worker does not send a `CAN_DO` after it has lost a connection to a gearman job server and then regained it. This means that if the job server dies unexpectedly, it does not remember the workers registered to it when it comes back online. (See meetings/gearsloth#31)

In this patch the worker will send a `RESET_ABILITIES` (in case the job server did not go down) followed by a `CAN_DO` on a successful reconnect.