PPLNS with job declaration

[Fi3]

@sjors

This takes time and bandwidth. And in order to verify if the new transaction is actually valid the JDS has to insert it into its mempool. And in order to do that it may need to evict other conflicting transactions first. Doing this for every DeclareMiningJob may not scale very well.

not the most scalable solution but is the best trade off that we have been able to find, and a pool can support it.