tinywallet: from pools to VSPs

[teknico]

The tinywallet package still uses the old pool terminology, from before the change to VSP (Virtual Service Provider), in code, user-visible text, docstrings and comment.

[JoeGruffins]

Voting Service Provider

I agree that pool is a bit misleading, since they are solo-staking. It's not like pooled mining, though it may be one day. And @bee wants this changed I think.