plugin: support dynamic enable/disable plugins

[lysu]

What problem does this PR solve?

• support dynamic disable/enable a loaded plugins.
• have ablity to observe plugin's enable/disable state
• enable/disable operation will affect all nodes in the cluster

What is changed and how it works?

need use parser's new syntax pingcap/parser#374

then just change current disable flag and broadcast change via etcd

attention: we also need update plugins to let plugin's go.mod dep on new parser...

Check List

Tests

• Manual test (add detailed scripts or steps below)

./bin/tidb-server -store tikv -path 127.0.0.1:2379 -plugin-dir=$GOPATH/src/github.com/pingcap/enterprise-plugin/audit -plugin-load=audit-1

mysql> show plugins;
+-------+--------------+-------+------------------------------------------------------------------------------+---------+---------+
| Name  | Status       | Type  | Library                                                                      | License | Version |
+-------+--------------+-------+------------------------------------------------------------------------------+---------+---------+
| audit | Ready-enable | Audit | /home/robi/code/go/src/github.com/pingcap/enterprise-plugin/audit/audit-1.so |         | 1       |
+-------+--------------+-------+------------------------------------------------------------------------------+---------+---------+
1 row in set (0.01 sec)

mysql> admin plugins disable audit;
Query OK, 0 rows affected (0.01 sec)

mysql> show plugins;
+-------+---------------+-------+------------------------------------------------------------------------------+---------+---------+
| Name  | Status        | Type  | Library                                                                      | License | Version |
+-------+---------------+-------+------------------------------------------------------------------------------+---------+---------+
| audit | Ready-disable | Audit | /home/robi/code/go/src/github.com/pingcap/enterprise-plugin/audit/audit-1.so |         | 1       |
+-------+---------------+-------+------------------------------------------------------------------------------+---------+---------+
1 row in set (0.00 sec)

mysql> admin plugins enable audit;
Query OK, 0 rows affected (0.00 sec)

mysql> show plugins;
+-------+--------------+-------+------------------------------------------------------------------------------+---------+---------+
| Name  | Status       | Type  | Library                                                                      | License | Version |
+-------+--------------+-------+------------------------------------------------------------------------------+---------+---------+
| audit | Ready-enable | Audit | /home/robi/code/go/src/github.com/pingcap/enterprise-plugin/audit/audit-1.so |         | 1       |
+-------+--------------+-------+------------------------------------------------------------------------------+---------+---------+
1 row in set (0.00 sec)

Code changes

• impl change

Side effects

• N/A

Related changes

• Need to cherry-pick to the release 3.0/2.1

---

This change is 

[codecov]

Codecov Report

Merging #11122 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #11122   +/-   ##
===========================================
  Coverage   81.3541%   81.3541%           
===========================================
  Files           421        421           
  Lines         90948      90948           
===========================================
  Hits          73990      73990           
  Misses        11702      11702           
  Partials       5256       5256

[lysu]

plugin build CI will be fixed after https://github.com/pingcap/enterprise-plugin/pull/8 be merged

[tiancaiamao]

LGTM

[lysu]

/run-all-tests

[jackysp]

LGTM

[jackysp]

/rebuild