tikv: collapse duplicate resolve locks in region requests (#16838)

[sre-bot]

cherry-pick #16838 to release-4.0

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What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

When txn1, txn2 meet txn1's lock, and txn1 is large txn that modify many keys

both txn1, txn2 will try to resolve(after check txn status) the whole region even if txn1 meet lock at k1 and txn2 meet lock at k2.

we can deduplicate resolve requests that try to resolve the whole region.

What is changed and how it works?

Proposal: xxx

What's Changed:

At first glance, we should do it in LockResolver, but the question is it's hard to maintain backoff behavior at LockResolver level:

if we collapse multiple lock requests into one request at LockResolver level, working request maybe meet many error and backoff(e.g. regionMiss, notLeader, kvRpc...), we are hard to let those backoff info feedback to collapsed requests that waiting for working request.

so this PR chooses to do it in lower level ---- at TiClient that this level, we only need to take care context.Cancel and timeout.

How it Works:

collapse multi resolve request into one request using singleflight.

and chose DoChan to impl request level timeout and context.Cancel

Related changes

• Need to cherry-pick to the release branch(after test..)

Check List

Tests

• Integration test(old test)

Side effects

• reduce kv resolve request count

Release note

collapse duplicate resolve locks in region requests

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This change is 

[sre-bot]

/run-all-tests

[tiancaiamao]

LGTM

[2020-04-29T02:45:24.414Z] ----------------------------------------------------------------------
[2020-04-29T02:45:24.414Z] FAIL: <autogenerated>:1: tidbTestSerialSuite.SetUpSuite
[2020-04-29T02:45:24.414Z] 
[2020-04-29T02:45:24.414Z] tidb_test.go:92:
[2020-04-29T02:45:24.414Z]     c.Assert(err, IsNil)
[2020-04-29T02:45:24.414Z] ... value *errors.withStack = listen tcp 0.0.0.0:10094: bind: address already in use ("listen tcp 0.0.0.0:10094: bind: address already in use")
[2020-04-29T02:45:24.414Z] 
[2020-04-29T02:45:24.414Z] 
[2020-04-29T02:45:24.414Z] ----------------------------------------------------------------------

[crazycs520]

LGTM

[lysu]

@jackysp need a approve and merge thx~

[jackysp]

/merge

[sre-bot]

/run-all-tests

[sre-bot]

@sre-bot merge failed.

[lysu]

/run-integration-copr-test