Optimize mysql consistency

[lichunzhu]

What problem does this PR solve?

fix #118

1. Now dumpling will use FTWRL lock tables from start to the end, which is not suitable.
2. Dumpling --threads argument can't control concurrently dumping tables.
3. ResolveAutoConsistency in adjustConfig will lead consistency to none because serverType is not got yet.

What is changed and how it works?

1. Change rateLimit to connectionsPool to control database connections like mydumper.
2. Add Start for TableIR. concurrentDumpTable will return an array of not started TableIRs.
3. Move ResolveAutoConsistency after detect ServerType.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Side effects

• Increased code complexity

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release note

• Release FTWRL after connections are created to reduce lock time for mysql.

[codecov]

Codecov Report

Merging #121 into master will decrease coverage by 1.43%.
The diff coverage is 35.18%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   52.19%   50.75%   -1.44%     
==========================================
  Files          17       17              
  Lines        1868     1909      +41     
==========================================
- Hits          975      969       -6     
- Misses        825      868      +43     
- Partials       68       72       +4     

[kennytm]

LGTM