Mempool persistence was added in
3f78562df5e86a2a0a21812047fc3a7db8cee988, and is always on. This commit
introduces a command-line parameter -persistmempool, which defaults to
true. When set to false:
- mempool.dat is not loaded when the node starts.
- mempool.dat is not written when the node stops.
Backport core's PR9966 and PR10342