Allows converting mapBlocksInFlight and g_last_tip_update from globals to member variables, and remove need to pass mempool or consensus params to methods.
This is a backport of core#20942 [4&5/9]
https://github.com/bitcoin/bitcoin/pull/20942/commits/a490f0a056456d683dd8ef6f89a5af1a13792118
https://github.com/bitcoin/bitcoin/pull/20942/commits/d44084883adcf00f50d3d5a9e0c88e3a0b276817
Note: `consensusParams` is not needed in our version of `FindNextBlocksToDownload`. Core uses it to test `IsWitnessEneabled`.
Depends on D11434