Add support for simulated mining

[larskuhtz]

The current devnet setup uses in-node mining. In node mining has the advantage that it is easy to scale the number of mining nodes by increasing the replica count.

Simulated mining requires the use of chainweb-mining-client. Chainweb-web-mining clients must be mapped to a single chainweb-node. The internal service routing of the docker compose network probably doesn't guarantee sticky application level sessions. If a service container "knows" its replica number, it could be used to assign it to a unique mining node. Alternatively, we could use the the replica count of the mining nodes to randomly assign mining-client to a unique node by hashing some unique identifier (e.g. the internal IP address) of the client.