Add support for extending TTR

[SamMousa]

TTR is a mechanic to release crashed / stalled jobs back into the queue.
Sometimes jobs are slow but not stalled.
Consider the DownloadJob from the docs; I could easily make this job exceed its TTR by throttling the connection at the server.
However since the process is alive and actually doing what it is supposed to do (assuming it is still actually receiving more than 0 b/s) we might want to tell the queue that we're still busy and it shouldn't release our job to someone else just yet.

Beanstalk protocol uses touch for this, and this shouldn't be hard to implement for other drivers.
The database driver currently doesn't do TTR at all (#249). When it does, all we need to do to extend the time is update the reserved_at field.
For other drivers I'm not sure how to implement this exactly, but I don't think it'll be hard.

[githubjeka]

Rel #249