Synchronize AccountLoader#get and #put

These methods are called from ChangeJson which runs formatting in
parallel. Since these methods modify local variables that are not
thread-safe, they need to be synchronized.

Both methods modify distinct variables so that an overall
synchronization is not required.

Change-Id: I328dc1265c3329f2c7ad66fb5946f30ca4e5f3b8

java/com/google/gerrit/server/account/AccountLoader.java

@@ -69,7 +69,7 @@ public class AccountLoader {
     provided = new ArrayList<>();
   }
 
-  public AccountInfo get(Account.Id id) {
+  public synchronized AccountInfo get(Account.Id id) {
     if (id == null) {
       return null;
     }
@@ -81,7 +81,7 @@ public class AccountLoader {
     return info;
   }
 
-  public void put(AccountInfo info) {
+  public synchronized void put(AccountInfo info) {
     checkArgument(info._accountId != null, "_accountId field required");
     provided.add(info);
   }