`SetNewPrevHash` message name exists in two separate protocols

[plebhash]

this is somewhat related to what has already been pointed out in #130

these messages achieve very similar purposes, but in slightly different ways... the Mining Protocol variant is always targeting one specific future job for activation, while the Template Distribution Protocol variant is always targeting one specific future template for activation

while having them share the same name kinda makes sense, it also brings a lot of potential for confusion (as we can see in #130)

I believe better names could be:

• ActivateFutureJob: which is the Mining Protocol variant
• ActivateFutureTemplate: which is the Template Distribution Protocol variant

[Fi3]

SetNewPrevHash is ok as name IMO, maybe we can do SetNewPrevHashTP and SetNewPrevHashMining

[Sjors]

SetTemplatePrevHash would make sense for the TP, and then maybe keep the name SetNewPrevHash for the mining protocol?