Re: [PATCH] Reuse Workers and Replication Slots during Logical Replication

1. In the function ApplyWorkerMain.
I think we need to keep the worker name as "leader apply worker" in the comment
like the current HEAD.

I think in this case we also need to pop the error context stack before
returning. Otherwise, I think we might use the wrong callback
(apply error_callback) after we return from this function.

3. About the function UpdateSubscriptionRelReplicationSlot.
This newly introduced function UpdateSubscriptionRelReplicationSlot does not
seem to be invoked. Do we need this function?

I think if 'need_full_snapshot' is false, it means we will create a snapshot
that can read only catalogs. (see SnapBuild->building_full_snapshot)

```
'use' will use the snapshot for the current transaction executing the command.
This option must be used in a transaction, and CREATE_REPLICATION_SLOT must be
the first command run in that transaction.
```

So I think in the function CreateDecodingContext, when "need_full_snapshot" is
true, we seem to need the following check, just like in the function
CreateInitDecodingContext:

```
        if (IsTransactionState() &&
                GetTopTransactionIdIfAny() != InvalidTransactionId)
                ereport(ERROR,
                                (errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
                                 errmsg("cannot create logical replication slot in transaction that has performed writes")));
``` 

It seems that we also need to invoke the function
CheckLogicalDecodingRequirements in the new function CreateReplicationSnapshot,
just like the function CreateReplicationSlot and the function
StartLogicalReplication.

3. The invocation of startup_cb_wrapper in the function CreateDecodingContext.
I think we should change the third input parameter to true when invoke function
startup_cb_wrapper for CREATE_REPLICATION_SNAPSHOT. BTW, after applying patch
v10-0002*, these settings will be inconsistent when sync workers use
"CREATE_REPLICATION_SLOT" and "CREATE_REPLICATION_SNAPSHOT" to take snapshots.
This input parameter (true) will let us disable streaming and two-phase
transactions in function pgoutput_startup. See the last paragraph of the commit
message for 4648243 for more details.