A few warning and log clean up:

• fix 2 build warning which is new imported
• change the boring repeat string StratumServer to 0, the id here is reserved for the case of multiple stratum server threads, and in Grin we only start one stratum server thread.

WARN grin_servers::mining::stratumserver - (Server ID: StratumServer) Solution Found for block

Now it's:

WARN grin_servers::mining::stratumserver - (Server ID: 0) Solution Found for block

• Add block hash in the solution found log, like Solution Found for block 2035, hash 08321e6c.