This is a step toward making the TxRequestTracker a template class able
to deal with transactions and avalanche proofs.
This is a massive renaming of the TxRequestTracker:
- tx => inv
- Tx => Inv
- (a) transaction => (an) inventory
- transactions => inventories
- Various minor fixups in comments
Note: the public names are not updated so the API is left unchanged and
there is no change in behavior.
Depends on D9559.