support external timestamp read

[YangKeao]

Enhancement

Add two more variables: global tidb_external_timestamp and global/session tidb_enable_external_timestamp_read.

When the tidb_enable_external_timestamp_read is on, use the tidb_external_timestamp as the timestamp to perform stale read.

The global variable tidb_external_timestamp should be stored in pd.

Senerio

This feature can be useful in the following senerio:

the data replication software (e.g. TiCDC) could use this variable to tell the downstream TiDB the data before which timestamp is safe to read. The client of the downstream TiDB could turn on tidb_enable_external_timestamp_read and read from the fully synced data.

[lance6716]

Can you explain a bit of the use case of this feature?

For example, I can imagine that some data replication software periodically updates tidb_external_timestamp when replicates data to TiDB, and some connections on TiDB turned on tidb_enable_external_timestamp_read to run queries on that "near real time" timestamp.

[YangKeao]

A lot of PRs about this feature should be merged in order:

1. cluster: support external timestamp tikv/pd#5631
2. implement external timestamp for mock pd tikv/client-go#611
3. unistore: implement external timestamp for mock pd #38689
4. implement the external timestamp client tikv/client-go#610
5. *: add variables and support external timestamp read #38276