[close #606]: change the default prefix for API v2

[iosmanthus]

Signed-off-by: iosmanthus [email protected]

What problem does this PR solve?

Issue Number: close #issue_number

Problem Description: Current key prefix of API v2 is not extensible enough, because no more keyspace information could be attached to it which is against the original design for the https://github.com/tikv/rfcs/blob/master/text/0069-api-v2.md.

What is changed and how does it work?

This pull request adds a reserved zero to represent the default keyspace.

Check List for Tests

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

• Unit test
• Integration test

[codecov]

Codecov Report

Merging #607 (53d8678) into master (9ce52d3) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #607      +/-   ##
============================================
- Coverage     34.60%   34.58%   -0.03%     
+ Complexity     1418     1415       -3     
============================================
  Files           278      278              
  Lines         17342    17344       +2     
  Branches       1970     1970              
============================================
- Hits           6002     5998       -4     
- Misses        10732    10741       +9     
+ Partials        608      605       -3     

Impacted Files	Coverage Δ
.../org/tikv/common/apiversion/RequestKeyV2Codec.java	82.85% <100.00%> (+1.03%)	⬆️
...g/tikv/common/apiversion/RequestKeyV2RawCodec.java	100.00% <100.00%> (ø)
...g/tikv/common/apiversion/RequestKeyV2TxnCodec.java	100.00% <100.00%> (ø)
src/main/java/io/grpc/netty/WriteQueue.java	74.43% <0.00%> (-2.26%)	⬇️
...rc/main/java/io/grpc/netty/NettyClientHandler.java	56.68% <0.00%> (-0.44%)	⬇️
...ty/handler/codec/http2/Http2ConnectionHandler.java	48.41% <0.00%> (-0.25%)	⬇️

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