Optimize JSON->MsgPack stream conversion

[xerial]

• Uses two-pass algorithm (fill empty MAP32/ARRAY32 headers in the first pass, then fill them with the real headers later after the size of arrays and maps can be determined). This will save one memory copy.
• Added json_stream benchmark to see the performance.
  • JSON -> JSONValue -> MsgaPack conversion is not so bad for small JSON
  • JSON -> two-pass algorithm -> MsgPack is a clear winner for large JSON. For a small single JSON object, no significant difference exists.

Benchmark                    Mode  Cnt  Score    Error  Units
IntArraySON.jsonValue        avgt   10  0.010 ±  0.001  ms/op
IntArraySON.nested           avgt   10  0.011 ±  0.001  ms/op
IntArraySON.twoPass          avgt   10  0.018 ±  0.001  ms/op
TwitterJSON.jsonValue        avgt   10  5.338 ±  0.914  ms/op
TwitterJSON.nested           avgt   10  8.017 ±  1.082  ms/op
TwitterJSON.twoPass          avgt   10  4.743 ±  1.117  ms/op
TwitterSingleJSON.jsonValue  avgt   10  0.029 ±  0.007  ms/op
TwitterSingleJSON.nested     avgt   10  0.046 ±  0.018  ms/op
TwitterSingleJSON.twoPass    avgt   10  0.026 ±  0.006  ms/op

[codecov]

Codecov Report

Merging #725 into master will decrease coverage by 0.52%.
The diff coverage is 85.91%.

@@            Coverage Diff             @@
##           master     #725      +/-   ##
==========================================
- Coverage   83.29%   82.76%   -0.53%     
==========================================
  Files         217      218       +1     
  Lines        8523     8588      +65     
  Branches      598      603       +5     
==========================================
+ Hits         7099     7108       +9     
- Misses       1424     1480      +56

Impacted Files	Coverage Δ
...main/scala/wvlet/airframe/msgpack/spi/Packer.scala	50% <ø> (ø)	⬆️
...rframe/msgpack/json/NestedMessagePackBuilder.scala	0% <0%> (ø)
...main/scala/wvlet/airframe/codec/MessageCodec.scala	84.31% <100%> (ø)	⬆️
...cala/wvlet/airframe/msgpack/spi/OffsetPacker.scala	98.13% <100%> (+0.02%)	⬆️
...scala/wvlet/airframe/msgpack/spi/WriteCursor.scala	93.33% <100%> (+0.47%)	⬆️
...scala/wvlet/airframe/msgpack/spi/MessagePack.scala	80% <100%> (ø)	⬆️
...scala/wvlet/airframe/msgpack/impl/PackerImpl.scala	64.44% <100%> (+0.8%)	⬆️
...rc/main/scala/wvlet/airframe/codec/JSONCodec.scala	89.58% <66.66%> (ø)	⬆️
...rframe/msgpack/json/StreamMessagePackBuilder.scala	89.65% <89.65%> (ø)
... and 1 more

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