Split mining into out-of-process stratum mining client

[yeastplume]

Basically,

• allow cuckoo-miner to run as a standalone client, and use it to interface with grin's new stratum server
• remove plugin-specific mining configuration from grin and insert into cuckoo miner

I've made a start in the stratum-client branch here: [WIP] https://github.com/mimblewimble/cuckoo-miner/tree/stratum-client

[yeastplume]

https://github.com/mimblewimble/grin/pull/979