[to #464] add unit tests for exported API of RawKVClient

[iosmanthus]

Signed-off-by: iosmanthus myosmanthustree@gmail.com

What problem does this PR solve?

Issue Number: to #464

Problem Description:

Some of the exported API of RawKVClient is not covered by unit tests and this blocks the compatibility tests. This pull request add more tests to cover these APIs including:

• batchDelete
• scan0
• TiKVIterator
• ingest

Check List for Tests

This PR has been tested by at least one of the following methods:

• Unit test
• Integration test

[zz-jason]

LGTM

@iosmanthus you can solve the issues in another pull request

[iosmanthus]

@marsishandsome PTAL

[codecov]

Codecov Report

Merging #471 (3b56877) into master (92fea32) will increase coverage by 0.35%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #471      +/-   ##
============================================
+ Coverage     32.59%   32.94%   +0.35%     
- Complexity     1324     1329       +5     
============================================
  Files           270      270              
  Lines         17054    17054              
  Branches       1950     1950              
============================================
+ Hits           5559     5619      +60     
+ Misses        10889    10829      -60     
  Partials        606      606              

Impacted Files	Coverage Δ
...ty/handler/codec/http2/Http2ConnectionHandler.java	48.65% <0.00%> (-5.14%)	⬇️
...rc/main/java/io/grpc/netty/NettyClientHandler.java	56.89% <0.00%> (-1.94%)	⬇️
src/main/java/io/grpc/internal/ClientCallImpl.java	56.34% <0.00%> (-0.74%)	⬇️
src/main/java/org/tikv/common/PDClient.java	59.27% <0.00%> (-0.49%)	⬇️
src/main/java/org/tikv/common/TiSession.java	70.67% <0.00%> (-0.49%)	⬇️
.../codec/http2/DefaultHttp2RemoteFlowController.java	59.31% <0.00%> (-0.39%)	⬇️
...va/org/tikv/common/region/StoreHealthyChecker.java	75.00% <0.00%> (+1.31%)	⬆️
.../main/java/org/tikv/common/region/RegionCache.java	50.92% <0.00%> (+1.85%)	⬆️
...java/org/tikv/common/region/RegionStoreClient.java	50.61% <0.00%> (+2.76%)	⬆️
src/main/java/org/tikv/raw/RawKVClient.java	73.42% <0.00%> (+11.59%)	⬆️
... and 1 more

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[marsishandsome]

LGTM

[zz-jason]

@iosmanthus please resolve conflicts.

[iosmanthus]

@zz-jason @marsishandsome PTAL

[zz-jason]

LGTM