disable mempool WAL by default (#2490)

CHANGELOG_PENDING.md

@@ -5,6 +5,7 @@ Special thanks to external contributors on this release:
 BREAKING CHANGES:
 
 * CLI/RPC/Config
+- [config] `mempool.wal` is disabled by default
 
 * Apps
 

config/config.go

@@ -503,7 +503,7 @@ func DefaultMempoolConfig() *MempoolConfig {
 		Recheck:      true,
 		RecheckEmpty: true,
 		Broadcast:    true,
-		WalPath:      filepath.Join(defaultDataDir, "mempool.wal"),
+		WalPath:      "",
 		// Each signature verification takes .5ms, size reduced until we implement
 		// ABCI Recheck
 		Size:      5000,

docs/tendermint-core/running-in-production.md

@@ -74,6 +74,10 @@ propose it. Clients must monitor their txs by subscribing over websockets,
 polling for them, or using `/broadcast_tx_commit`. In the worst case, txs can be
 resent from the mempool WAL manually.
 
+For the above reasons, the `mempool.wal` is disabled by default. To enable, set
+`mempool.wal_dir` to where you want the WAL to be located (e.g.
+`data/mempool.wal`).
+
 ## DOS Exposure and Mitigation
 
 Validators are supposed to setup [Sentry Node