Special Interest Groups should never divide repository

[tisonkun]

subtask of #516

Generally SIGs own one or more repository, but sig-sql-infra, sig-planner, sig-execution, sig-migration, and so on share the main repo pingcap/tidb.

It hurts that we artificially break GitHub repo into several parts while database internals are interrelated. Most of our contributors are looking forward to a GitHub repo granularity governance that can be easily understood.

I propose to redeclare SIGs to meet a GitHub repo granularity governance that can be easily understood.

Creating a pull request with a detail membership list...

[tisonkun]

A new sig (named as team for distinguished) has its reviewer, committer and maintainer.

• A reviewer is able to review pull request for quality and correctness of repositories associated with the team. His/Her LGTM is counted.
• A committer is able to both review and commit pull request.
• A maintainer, based on the rights of a committer, is able to promote or evict(emeritus member for reference) a reviewer, a committer or a maintainer by majority decision.

The maintainers is able to evict member by majority decision and thus we have an exit mechanism. It is suggested that a team defines its own bylaws to elaborate other team level decision making process.

Besides the teams, the whole TiDB community has its TOC members for evolving the community. The TOC members are in charge of community level decision making process.