Allows making mapRelay and vRelayExpiration members rather than globals.
Remove need to pass mempool of conman to methods?
This is a backport of core#20942 [6&7/9]
https://github.com/bitcoin/bitcoin/pull/20942/commits/34207b9004d2069a8fcb32758cd796143eccfb4d
https://github.com/bitcoin/bitcoin/pull/20942/commits/7b7117efd00acf7609e65d3b4fe5f76e400dda12
Depends on D11435
Notes :
- also convert a uint256 to TxId in mapRelay type
- rename g_relay_expiration to vRelayExpiration now that it is no longer a global (undoes a change in D10914). The m_ prefix is not very commonly used in this class, so I feel like it is best to stick with the source material.