Currently, the mempool tracks fees that were manually set by miners using the rpc interface.
These values are not added to CBlockTemplateEntry currently, as they are not used for anything
outside of prioritizing transactions.

However, future commits will move from using the MempoolEntry for adding transactions into a block
and directly use CBlockTemplateEntry. So, we need to track modified fees there as well for sorting purposes.
The primary field (txFee) is left alone, as it's sum must + reward must be a valid coinbase output value.

Depends on D2865