Support cache rows in memory for each region

[humengyu2012]

What is the purpose of the change

(For example: This pull request makes task deployment go through the blob server, rather than through RPC. That way we avoid re-transferring them on each deployment (during recovery).)

Brief change log

1. Support cache rows in memory for each region, and add a new config named tidb.cached-rows-in-memory to enable or disable it.
2. Add a wrap for CoprocessorIterator, and it could retry by each role for reading data.

Verifying this change

This change added tests and can be verified as follows:

Add some tests in io.tidb.bigdata.tidb.MemoryIteratorTest.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The runtime per-record code paths (performance sensitive): yes

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs

[humengyu2012]

/run-all-tests