The *_RECENT_CONSENSUS_CHANGE variants in the validation result
enumerations were always unused. They seem to have been kept around
speculatively for a soft fork after segwit, however they were never used
for taproot either. This points at them not having a clear purpose.
Based on the original pull requests' comments their usage was never
entirely clear:
https://github.com/bitcoin/bitcoin/pull/11639#issuecomment-370234133https://github.com/bitcoin/bitcoin/pull/15141#discussion_r271039747
Since they are part of the validation interface and need to exposed by
the kernel library keeping them around may also be confusing to future
users of the library.
Previously this assertion checked MAX_PEER_TX_REQUEST_IN_FLIGHT was not
exceeded. However, this property is not actually enforced; it is just
used to determine when a peer is overloaded.
The txdownload_impl is similar but allows us to check specific
invariants within its implementation. It will also change a lot more
than the external interface (txdownloadman) will, so we will add more to
this target later.
