*: add HIGH_PRIORITY for the bootstrap SQLs.

[jackysp]

What problem does this PR solve?

Some important SQLs could failed when TiDB is starting. This PR try to improve this.

What is changed and how it works?

Add HIGH_PRIORITY for the bootstrap SQLs.

Check List

Tests

• Manual test (add detailed scripts or steps below)

1. change the log level to debug, add os.Exit(0) after

   tidb/server/server.go

   Line 241 in 1a97152

   for {

2. start a cluster with 2 TiDB servers.
3. check the SQLs in the two log files.

Code changes

• no

Side effects

• Possible performance regression

Related changes

• no

PTAL @winkyao @breeswish

[jackysp]

/run-all-tests

[winoros]

This change is made by gofmt?

[jackysp]

Yes.

[tiancaiamao]

LGTM

[XuHuaiyu]

According to @hicqu , this PR could not solve the problem described in the description.

SQL with HIGH_PRIORITY would not be promised to be executed before the NORMAL_PRIORITY now.

[XuHuaiyu]

I got it, HIGH_PRIORITY task can be handled in a short time if there are few HIGH_PRIORITY tasks in the task queue, rather than waiting until out of time when be set as NORMAL_PRIORITY.

[zz-jason]

LGTM