Reduce RPC for point get

[you06]

Enhancement

TiDB executes point-get requests with non-clustered indexes in the following process:

• Read index value
• Lock index key
• Lock handle key

An ideal process may be reading the index value from the locking response. But for RC mode, we only lock keys when they exist, there may be a lock-if-exist option for the pessimistic lock.

Another possible solution is the batch-point-get's style, locks index keys and handle keys in parallel, which does not follow a fixed locking order, and may be more risk of deadlock.

[you06]

RC mode optimization is done by #36834, Close this issue.