[Privacy] Stratum user maps

[1ma]

When renting hashrate it's currently easy for the hash provider to figure out the end user on the OCEAN dashboard. Just needs to wait 3h and look up for miners on the range with the same workername that was handed to him.

For these cases the DATUM gateway could do optional silent translation of specific Stratum workernames. The stratum section of the configuration could define an optional config object like so:

"user_map": {
  "foo": "bar"
}

Any client that identifies as "foo" would appear as "bar" in the OCEAN dashboard. This should only affect the workername part of the Stratum username (the part after the dot), and shouldn't affect clients whose workername doesn't appear in the user map.