domain: lock reload and fix version check.

 In mysql test , sometimes the domain’s schema version  may be changed
lesser because of another slow transaction.
So we must use lock to guarantee reload once at same time and update
the version check way.
It is very hard to write a test for this, so if we can pass mysql test
using HBase with no error, I think it is fixed.

domain/domain.go

@@ -14,6 +14,7 @@
 package domain
 
 import (
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -40,6 +41,7 @@ type Domain struct {
 	leaseCh    chan time.Duration
 	// nano seconds
 	lastLeaseTS int64
+	m           sync.Mutex
 }
 
 func (do *Domain) loadInfoSchema(txn kv.Transaction) (err error) {
@@ -50,7 +52,9 @@ func (do *Domain) loadInfoSchema(txn kv.Transaction) (err error) {
 	}
 
 	info := do.infoHandle.Get()
-	if info != nil && schemaMetaVersion > 0 && schemaMetaVersion == info.SchemaMetaVersion() {
+	if info != nil && schemaMetaVersion <= info.SchemaMetaVersion() {
+		// info may be changed by other txn, so here its version may be bigger than schema version,
+		// so we don't need to reload.
 		log.Debugf("schema version is still %d, no need reload", schemaMetaVersion)
 		return nil
 	}
@@ -139,6 +143,10 @@ func (do *Domain) tryReload() {
 }
 
 func (do *Domain) reload() error {
+	// lock here for only once at same time.
+	do.m.Lock()
+	defer do.m.Unlock()
+
 	err := kv.RunInNewTxn(do.store, false, do.loadInfoSchema)
 	if err != nil {
 		return errors.Trace(err)