conf: rewrite unified pool related configurations

[sticnarf]

What problem does this PR solve?

Problem Summary:

When TiKV starts, it prints out the configuration in the log. Currently, if use-unified-pool is not set, null will be outputted. This behavior may be confusing to users and the users can fail to know the default setting TiKV uses.

What is changed and how it works?

What's Changed:

Instead of silently using default settings in TiKV, this PR rewrites use-unified-pool with the default setting. Now TiKV prints out the real setting of use-unifed-pool in the start log.

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

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

Now, even if use-unified-pool is not set, the real setting will be printed out like this: (previously readpool.*.use-unified.pool = null)

[2020/05/26 16:07:59.257 +08:00] [INFO] [server.rs:213] ["using config"] [config="{\"cfg-path\":\"conf/tikv.toml\",\"log-level\":\"info\",\"log-file\":\"/home/tidb/tidb-deploy/tikv-20160/log/tikv.log\",\"slow-log-file\":\"\",\"slow-log-threshold\":\"1s\",\"log-rotation-timespan\":\"1d\",\"log-rotation-size\":\"300MiB\",\"panic-when-unexpected-key-or-data\":false,\"readpool\":{\"unified\":{\"min-thread-count\":1,\"max-thread-count\":4,\"stack-size\":\"10MiB\",\"max-tasks-per-worker\":2000},\"storage\":{\"use-unified-pool\":true,\"high-concurrency\":6,\"normal-concurrency\":6,\"low-concurrency\":6,\"max-tasks-per-worker-high\":2000,\"max-tasks-per-worker-normal\":2000,\"max-tasks-per-worker-low\":2000,\"stack-size\":\"10MiB\"},\"coprocessor\":{\"use-unified-pool\":true,\"high-concurrency\":9,\"normal-concurrency\":9,\"low-concurrency\":9,\"max-tasks-per-worker-high\":2000,\"max-tasks-per-worker-normal\":2000,\"max-tasks-per-worker-low\":2000,\"stack-size\":\"10MiB\"}}...

Release note

Prints out the factual use-unified-pool configuration in the startup log.

[sre-bot]

/run-all-tests

[sticnarf]

/run-cherry-picker

[sre-bot]

cherry pick to release-4.0 in PR #7985