RE: Assertion failure in SnapBuildInitialSnapshot()

Dear Sawada-san,

Thank you for making the patch! I'm still considering whether this approach is
correct, but I can put a comment to your patch anyway.

```
-    Assert(!already_locked || LWLockHeldByMe(ProcArrayLock));
-
-    if (!already_locked)
-        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+    Assert(LWLockHeldByMe(ProcArrayLock));
```

In this function, we regard that the ProcArrayLock has been already acquired as
exclusive mode and modify data. I think LWLockHeldByMeInMode() should be used
instead of LWLockHeldByMe().
I confirmed that there is only one caller that uses ReplicationSlotsComputeRequiredXmin(true)
and it acquires exclusive lock correctly, but it can avoid future bug.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED