Refine BuildNextBlock transaction selection logic

Currently, BuildNextBlock just adds every transaction to the block, which will be problematic when it's passed any conflicting transactions. We need to instead apply transactions based on feerate, weight, and pegin fees.

Locate the BuildNextBlock implementation and inspect how it currently adds transactions to a block. Trace the transaction and block-validation tests or entry points that cover conflicting transactions, feerate, weight, and pegin fees. Done means conflicting transactions are selected according to those criteria rather than all being added indiscriminately.

Written by the indexing model from the issue text.