Make token-limit configurable as an INSTANCE scoped variable

## Enhancement

This is forked from https://github.com/pingcap/tidb/issues/33769

For `token-limit` there is a use-case to make it online configurable, but it's not a great fit for `GLOBAL`. This enhancement proposes that we add an instance scoped variable for it.

For the name, I suggest `tidb_max_connection_concurrency` or `tidb_connection_concurrency_limit`.

This is different from the issues in https://github.com/pingcap/tidb/issues/33769 because it is not a breaking change. It can still be configured in the config file as `token-limit` or via the instance scoped variable that we are introducing.