Iterator performance

[stroomdev66]

This is just a draft for now to show iterator performance improvements when using specific classes for cursor direction etc and removing the state machine approach.

Closes gh-269

[stroomdev66]

This is just a draft for discussion. If the approach is accepted then the current iterator would be replaced to maintain API compatability.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 71.86184% with 334 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 81.31%. Comparing base (65df2ee) to head (9541da3). :warning: Report is 98 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/java/org/lmdbjava/DirectBufferProxy.java	25.42%	42 Missing and 2 partials :warning:
src/main/java/org/lmdbjava/Env.java	52.68%	38 Missing and 6 partials :warning:
src/main/java/org/lmdbjava/Dbi.java	56.97%	31 Missing and 6 partials :warning:
src/main/java/org/lmdbjava/KeyRange.java	68.68%	31 Missing :warning:
src/main/java/org/lmdbjava/ByteBufferProxy.java	54.83%	18 Missing and 10 partials :warning:
src/main/java/org/lmdbjava/LmdbIterable.java	82.87%	17 Missing and 8 partials :warning:
src/main/java/org/lmdbjava/ByteArrayProxy.java	4.34%	22 Missing :warning:
src/main/java/org/lmdbjava/ByteBufProxy.java	43.58%	18 Missing and 4 partials :warning:
src/main/java/org/lmdbjava/LmdbStream.java	85.21%	14 Missing and 7 partials :warning:
src/main/java/org/lmdbjava/Cursor.java	48.14%	8 Missing and 6 partials :warning:
... and 12 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #270      +/-   ##
============================================
- Coverage     89.06%   81.31%   -7.76%     
- Complexity      413      598     +185     
============================================
  Files            32       57      +25     
  Lines          1482     2531    +1049     
  Branches        125      279     +154     
============================================
+ Hits           1320     2058     +738     
- Misses           92      336     +244     
- Partials         70      137      +67     

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