`GeTSAsync` may be blocked

[CabinfeverB]

Bug Report

What did you do?

Due to thathandleDispatcher will be canceled after closing, so

pd/client/tso_dispatcher.go

Line 86 in 2e3e4c0

dispatcher.(*tsoDispatcher).tsoBatchController.tsoRequestCh <- request

will be blocked.

see new test case in #7862

What did you expect to see?

What did you see instead?

What version of PD are you using (pd-server -V)?

[CabinfeverB]

Because size of channel is large, so we can regard this as minor issue.