Handle failover by catching SIGUSR1 signal AND other refactors

based off of !1339 (diffs)

addresses: #1615 (closed)

This change will catch SIGUSR1, and attempt to promote the next secondary to primary, and update the datastore so that the new primary gets used on proxy requests, and also the secondary that got promoted no longer gets replication jobs assigned to it.

Closes #1723 (closed)

Refactor

• moved Repository, GitalyServer to package models