Update stratum server code uses of global "edge_bits" value

[bladedoyle]

This does not look right to me:
https://github.com/mimblewimble/grin/blob/master/servers/src/mining/stratumserver.rs#L635

It does not make sense to have a single stratum-wide value for "edge_bits" (or cuckoo_size) now that we allow multiple sizes chosen by the miner on a per solution basis.

It looks like all the "status" stuff needs to be updated also

Does a single "minimum_share_difficulty" setting applied to all solution sizes (in the grin-server.toml) still make sense?

[bladedoyle]

I disagree with the "enhancement" tag. Adding multiple POW was an enhancement. Unmade changes needed as a result of that enhancement is a bug/technical debt. (imo at least)

[lehnberg]

Apologies, I read it as though something additional was needed beyond what was already in there. Not that previous functionality broke as part of changes. Updated.

[yeastplume]

I think we're okay here now