mempool: Stake-related readability improvements.
mempool: Stake-related readability improvements.

This does some minor cleanup of the code in the maybeAcceptTransaction
function to use some local bools with more easily readable names for
dealing with stake transactions.

It also improves the readability of the double spending exceptions
section and avoids some additional if statements.

decred/dcrd,decred/dcrd,jrick/dcrd,jrick/dcrd