config: synchronize actual default value with the toml file

[kennytm]

What problem does this PR solve?

Make the default values when missing a config file to be equivalent to the default TOML file.

Exposed a few settings in command line.

What is changed and how it works?

Changed default values:

• checkpoint.enable = true
• post-restore.analyze = true
• csv.header = true
• csv.null = '\n'
• csv.backslash-escape = true

Exposed global / command line options:

• --no-schema=false
• --enable-checkpoint=true
• --analyze=true
• --checksum=true
• --check-requirements=true

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)
  • inspect the config when running bin/tidb-lightning --analyze=true, bin/tidb-lightning --analyze=false, etc.

Side effects

• Breaking backward compatibility
  • default value with no config value is changed.

Related changes

• Need to update the documentation

[kennytm]

/run-all-tests

[kennytm]

PTAL @GregoryIan

[kennytm]

/run-all-tests

[kennytm]

Ping @GregoryIan

[kennytm]

PTAL @WangXiangUSTC @3pointer

[lance6716]

LGTM (from community)

[kennytm]

/run-all-tests

(Exposed --check-requirements to command line too.)