Fix resolution result retrieval in dep diff computation
Speed up scheduler query
Rework resolver tests

Now they are more like integration tests that use actual repos.
Rework dependency processing - do the computation in Python

Dependency difference computation used to be done in DB, which turned out to be
slow. This could be faster and also gets rid of cleanup routine, because
dependencies are saved only for builds, not for all resolutions.
Generalize keyed set difference
Improve resource reuse when processing multiple builds
Move event to separate module
Synchronize message sending with session state

This means that fedmsg messages are only sent after succesful commit.
More joinedloads in resolver
Temporarily disable package state watching in resolver
Log times of repo generation
Skip IOErrors on broken cached repos
Delete existing dependency changes when resolving
Lower the depth of distance computation

Improves performance a lot.
Don't try to insert empty depchange sets
Use nulls last when sorting by dependency distance

Previous output was kinda useless because the most important deps ended up in
the middle of the list
Conditionalize old depchange deletion
Fix depchange iteration in package-detail
Skip unavailable repos