parser : support drop sequence syntax.

[AilinKid]

What problem does this PR solve?

TiDB are compatible with MySQL's protocol, we should be also compatible with SEQUENCE implementation of MariaDB cause MySQL is absent from it.

•  DROP [TEMPORARY] SEQUENCE [IF EXISTS] sequence_name [, sequence_name] ...
  

Linked with sequence project, this PR try to support drop sequence syntax.

What is changed and how it works?

• SHOW CREATE SEQUENCE sequence_name;
• ALTER SEQUENCE [IF EXISTS] sequence_name [ INCREMENT [ BY | = ] increment ] [ MINVALUE [=] minvalue | NO MINVALUE | NOMINVALUE ] [ MAXVALUE [=] maxvalue | NO MAXVALUE | NOMAXVALUE ] [ START [ WITH | = ] start ] [ CACHE [=] cache ] [ [ NO ] CYCLE ] [ RESTART [[WITH | =] restart];
• DROP [TEMPORARY] SEQUENCE [IF EXISTS] sequence_name [, sequence_name] ...
• Sequence Function (need discussion)

Check List

Tests

• Unit test
• Integration test

[codecov]

Codecov Report

Merging #680 into master will increase coverage by 0.3%.
The diff coverage is 75%.

@@ Coverage Diff @@## master #680 +/- ##
=========================================
+ Coverage 79.82% 80.13% +0.3% 
=========================================
Files 37 33 -4 Lines 13596 13030 -566 =========================================
- Hits 10853 10441 -412 + Misses 2100 1967 -133 + Partials 643 622 -21

Impacted Files	Coverage Δ
parser.go	93.15% <ø> (ø)	⬆️
ast/ddl.go	79.67% <75%> (-0.07%)	⬇️
test_driver/test_driver_helper.go
test_driver/test_driver_datum.go
test_driver/test_driver_mydecimal.go
test_driver/test_driver.go
ast/dml.go	74% <0%> (+0.07%)	⬆️

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[kennytm]

Rest LGTM

[tangenta]

LGTM