Update partition processor to use Bifrost for reading commands

[tillrohrmann]

In order to move to the shared log architecture we need to update the partition processor to read its commands from Bifrost. This will allow us to remove the consensus module completely. Since the current Bifrost implementation does not support durability, we will sacrifice durability of the system until we have a durable implementation of the log. Additionally, we need to update the different writers (shuffle, partition processor itself and ingress) to use the log for sending partition processor commands.

[tillrohrmann]

Fixed via #1253