Refine the behavior of @@txn_scope value and Local Txn

[JmPotato]

Feature Request

Now, if we configure the zone label for a TiDB instance, then the default value of @@txn_scope will be "local", which is not quite reasonable. Because:

• User may only want to use the zone label to make sure the stale read could read the nearest data without enabling the Local TSO feature for the PD.
• Labeling a TiDB instance with a zone label without configuring the PD to enable Local TSO will get a PD timeout error because @@txn_scope is "local" by default, which is confusing.

So I suggest setting the default value of @@txn_scope to "global".

[JmPotato]

/assign @JmPotato

[nolouch]

@JmPotato I think the main reason is that @@txn_scope at the session level. It is unreasonable to set the @@txn_scope for each connection in the application if the local transaction is used, so we directly use local in default if exists label configuration in TiDB. At that time, only local transactions could be used with label configuration. I think maybe we need a global variable to control the default value of local/global transaction's txn scope. cc @Yisaer

[tisonkun]

Is this issue part of #21094?