Implemented support for external work server

[Joohansson]

• Ability to use an external work server defined in pow_creds.json
• Works together with boomPow, dpow with following fallback order: bpow > dpow > work server
• If enabled, it will NOT fallback to node work generation. If all bpow,dpow and work server is disabled: The node will be used
• Several work peers/servers can also be used if they are defined in the node config by setting use_work_peers to true. But only if bpow,dpow and work server are disabled
• Multiple work servers would have been nice (without using the node) but I think another decoupled application needs to be developed for that. That properly handles a work queue and optimized ordering of GPUs with different performance. Similar to how dpow works but better.

Fixes #83

[aspic]

Looks good! I think the logic of dpow/bpow/work server from line 696 could use some cleanup soon, but easier to do once this is in :-)