The TxRequestTracker is a well encapsulated class for managing the tx
inv response and scheduling of the associated getdata requests.

The proof inv can benefit from the exact same feature set. By making
this class a template depending on the inv id type (TxId or ProofId),
this will make it easy. All the specifics are left to the network layer.

Due to the template nature of the class, all the definitions are now in
the header file and the source file is no longer needed.

There is no change in behavior. The tests are updated in order to keep
the changes as minimal as possible.

Depends on D9554.