lightning: allocate autoid globally across all tidb instances

[sleepymole]

Enhancement

We are discussing the local backend below.

In #35148, we supported disabling pd scheduler by key range, so that we can import data to a table without significant impact on other tables which are used by online services.

Further, we want to support partition-level online import. That is to say, lightning can import data to a specific partition. Meanwhile, any read/write operations to other partitions on the same table are allowed and safe. There is no data loss or data corruption. To achieve this goal, we must avoid any autoid conflicts, because lightning bypasses the transaction layer, and any write conflicts will be ignored silently.