Control mempool persistence using a command line parameter.

Summary:
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

Test Plan:

make check
./test/functional/test_runner.py mempool_persist.py

Reviewers: #bitcoin_abc, dagurval

Reviewed By: #bitcoin_abc, dagurval

Differential Revision: https://reviews.bitcoinabc.org/D964