*: avoid accessing internal ports when backend=tidb

[kennytm]

What problem does this PR solve?

Fix #306.

The PD address port (:2379) and TiDB status port (:10080) by default are not exposed to public. In TiDB-Backend mode, we may not be able to access these ports.

What is changed and how it works?

Eliminated access of these two ports in TiDB backend by:

• Check-requirements are now a backend-specific operation, and is a no-op in TiDB backend (the "local" backend WIP backend: add local backend #294 should check the the TiKV version is 4.0.0, BTW.)
• The TiDB backend only needs to know the column names, so fetching the entire TableInfo is unnecessary. Moved the "fetch table info" operation into backend as well, where the TiDB backend simply fill in the column names from INFORMATION_SCHEMA.

Check List

Tests

• Unit test

Side effects

Related changes

• Need to be included in the release note
  • Lightning in TiDB-backend no longer requires access to PD or the TiDB status port. Thus, Lightning in TiDB-backend can be executed outside of the cluster if desired.

[lichunzhu]

LGTM

[3pointer]

LGTM