Rename tx to inv in the TxRequestTracker

Summary:
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 D9563.

Test Plan:

ninja all check-all
./test/functional/test_runner.py p2p_tx_download

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9560