Implement worker fork

[afsalthaj]

No description provided.

[vigoo]

I will do a proper review later, but some initial mismatch from the ticket:

• The forking logic should not be implemented in the gRPC request handlers. That should just be a thin layer as we want to have the actual forking logic used later in the debugging service for example. So as I wrote in the ticket, find a place (one of the "service layers" or create a new one) for it and from the gRPC request handler just call it.
• The ticket explicitly asks for opening the worker in suspended mode and use the Oplog instance of it to read the source oplog - and not the "global" one in OplogService. I am not sure exactly why did I ask for that but I suspect there was a reason :) By using Oplog it is properly synchronized in case the worker is running etc. So let's just do it in the way as the ticket asks.

[afsalthaj]

I used get_or_create_suspended and used oplog from there. If you look at some previous commits you can see that to later change to this one - but I thought it's the same. Anyway I will revert.