Fixed leaky buffer when streaming.

[svobol13]

Reworked a way how internal buffer size increases when streaming. The new way changes both when and how it increases size.

When: Whenever current buffer size is fully utilized AND autoresizing is enabled.
How: It increases buffer size with configured absolute number of bytes instead of doubling the capacity.

New API: Added new factory method parse(InputStream in, int bufSize, int autoExpandBufferStep) that creates Iterator with posibility to control how autoresizing works. Set 0 to disable, set 5 to increase buffer size of 5 bytes everytime we hit end.

Backward compatibility: Factory parse(InputStream in, int bufSize) was changed with respect to previous setting. First increase of buffer size is exactly same as it was in pass though any subsequent increase is same size as first one in contrast to previous behaviour (it was growing exponentially).

[codecov-io]

Codecov Report

Merging #243 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #243      +/-   ##
=========================================
+ Coverage   68.46%   68.5%   +0.04%     
=========================================
  Files         107     107              
  Lines        7335    7338       +3     
  Branches     1388    1389       +1     
=========================================
+ Hits         5022    5027       +5     
+ Misses       1867    1866       -1     
+ Partials      446     445       -1

Impacted Files	Coverage Δ
src/main/java/com/jsoniter/IterImpl.java	66.11% <ø> (ø)	⬆️
...c/main/java/com/jsoniter/IterImplForStreaming.java	70.73% <100%> (+0.46%)	⬆️
src/main/java/com/jsoniter/JsonIterator.java	65.54% <100%> (+0.52%)	⬆️

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