kvserver: reset some fields of replicaAppBatch

This commit fixes a bug in replicaAppBatch initialization. Some fields
were not reset, and would be carried across batches. For example, the
changeTruncatesSideloadedFiles field, once set to true, would be true
forever, and cause Pebble sync on every command application.

Epic: none
Release note: none

Author: pav-kv

pkg/kv/kvserver/replica_application_state_machine.go

@@ -139,6 +139,9 @@ func (sm *replicaStateMachine) NewEphemeralBatch() apply.EphemeralBatch {
 func (sm *replicaStateMachine) NewBatch() apply.Batch {
 	r := sm.r
 	b := &sm.batch
+	// TODO(pav-kv): replicaAppBatch initialization below is bug-prone, we need to
+	// not forget resetting the fields that are local to one batch. Find a way to
+	// make it safer.
 	b.r = r
 	b.applyStats = &sm.applyStats
 	b.batch = r.store.TODOEngine().NewBatch()
@@ -148,6 +151,9 @@ func (sm *replicaStateMachine) NewBatch() apply.Batch {
 	*b.state.Stats = *r.mu.state.Stats
 	b.closedTimestampSetter = r.mu.closedTimestampSetter
 	r.mu.RUnlock()
+	b.changeRemovesReplica = false
+	b.changeTruncatesSideloadedFiles = false
+	// TODO(pav-kv): what about b.ab and b.followerStoreWriteBytes?
 	b.start = timeutil.Now()
 	return b
 }