Bug 1093759 - Use fast-forward when landing from integration branch +autoland

This adds fast-forward support. Whenever a taskgraph passes we fast-forward the target branch to the passing commit. This adds support for a few things:

* Adds logic for notifying all bugs which were coalesced into a successful landing.
* Adds retry logic in the case we can't fast-forward. In this case we re-create the integration branch, and retry all pending jobs on top of the integration branch.

https://bugzilla.mozilla.org/show_bug.cgi?id=1093759

Hrm- lets chat about this a bit... Are we sure we want this in memory?