change DEFAULT_MAX_MEMPOOL_SIZE to 300(MB) (#411)

issue #410

src/policy/policy.h

@@ -27,7 +27,7 @@ static const unsigned int MAX_P2SH_SIGOPS = 15;
 /** The maximum number of sigops we're willing to relay/mine in a single tx */
 static const unsigned int MAX_STANDARD_TX_SIGOPS_COST = MAX_BLOCK_SIGOPS_COST/5;
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
-static const unsigned int DEFAULT_MAX_MEMPOOL_SIZE = 5;
+static const unsigned int DEFAULT_MAX_MEMPOOL_SIZE = 300;
 /** Default for -incrementalrelayfee, which sets the minimum feerate increase for mempool limiting or BIP 125 replacement **/
 static const unsigned int DEFAULT_INCREMENTAL_RELAY_FEE = 1000;
 /** Default for -bytespersigop */