mounter(ticdc): fix mounter default value panic and data inconsistency (#3930)

[ti-chi-bot]

This is an automated cherry-pick of #3930

close #3929 #3918

What problem does this PR solve?

When meet "amend + add column datetime default '2020-10-10 10:10:10' (DDL)", TiCDC will

1. panic, as the default value type is not consistent with column type
2. data inconsistence

What is changed and how it works?

1. Don't format default value in column meta data
2. Check default value exist before check not null flag

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)
  Start 2 transactions:

1. Transaction 1:

Create table test_column(id int primary key);
Insert Into test_column values(1);
begin;
Insert Into test_column values(2);
// hold the trx untill Transaction 2 commit DDL
commit;

• Transaction 2:

alter table test_column add column c0 datetime default "2020-10-10 10:10:10";

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update key monitor metrics in both TiCDC document and official document

Release note

Fix column default value panic and data inconsistency

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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