Control mempool persistence using a command line parameter.

Mempool persistence was added in
3f78562df5, 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.

src/validation.h

@@ -131,7 +131,8 @@ static const bool DEFAULT_PERMIT_BAREMULTISIG = true;
 static const bool DEFAULT_CHECKPOINTS_ENABLED = true;
 static const bool DEFAULT_TXINDEX = false;
 static const unsigned int DEFAULT_BANSCORE_THRESHOLD = 100;
-
+/** Default for -persistmempool */
+static const bool DEFAULT_PERSIST_MEMPOOL = true;
 /** Default for -mempoolreplacement */
 static const bool DEFAULT_ENABLE_REPLACEMENT = true;
 /** Default for using fee filter */