Support SHUTDOWN SQL command (and protocol cmd)

[liubo0127]

There is no smooth way to stop tidb and must stop the process by kill

[shenli]

OK, we will provide a way to shutdown tidb-server gracefully.

[morgo]

I suggest supporting the MySQL way: a shutdown api command (that mysqladmin can call) and an sql statement used in newer versions (shutdown). MySQL 8.0 also has a restart command.

[shenli]

It is hard for tidb-server to stop all the cluster instances.

[morgo]

I think the semantic expected is just for a single instance shutdown. I looked into this the other day, and the logical way seemed to do it was for SHUTDOWN or the protocol cmd ComShutdown to send an interrupt signal to the process.

The signal will then be caught by the main package, and start shutdown (not-withstanding a current issue that garbage collection doesn't like to be interrupted).

Thus, SHUTDOWN is just a nicer version of a Control+c to the window running tidb-server.

[ekalinin]

@morgo

Both PRs were merged (parser + tidb).

[morgo]

/cc @SunRunAway ^^

[SunRunAway]

Thanks.