Add split with limit operator (to protect against long input without delimiters)

[davidmoten]

Add Transformers.split(maxItemLength, pattern, maxPatternLength) operator.

This is a useful operator in the circumstance that the input is not neccessarily controlled and there exists the possibility of a really long stream of strings arriving without a delimiter. This may cause the normal split operator to throw an OutOfMemoryException as it accumulates a really long string waiting for the delimiter.

Unit test coverage of new operator is complete but for some isUnsubscribed() checks. These are painful to write tests for because buffering is occurring in the state-machine operator so I've left them out.

See javadoc of new operator for more detail.

In theory there is extra allocation overhead using this operator but benchmarks show no significant difference.

[codecov-io]

Codecov Report

Merging #34 into master will decrease coverage by 0.06%.
The diff coverage is 81.96%.

@@             Coverage Diff              @@
##             master      #34      +/-   ##
============================================
- Coverage     68.74%   68.68%   -0.07%     
- Complexity      417      427      +10     
============================================
  Files            69       69              
  Lines          3558     3602      +44     
  Branches        390      402      +12     
============================================
+ Hits           2446     2474      +28     
- Misses          988      996       +8     
- Partials        124      132       +8     

Impacted Files	Coverage Δ	Complexity Δ
.../rx/internal/operators/TransformerStringSplit.java	80.95% <81.35%> (+5.95%)	10.00 <10.00> (+9.00)
...in/java/com/github/davidmoten/rx/Transformers.java	75.72% <100.00%> (+0.28%)	45.00 <2.00> (+2.00)
...tors/QueueWithResourcesNonBlockingUnsubscribe.java	66.66% <0.00%> (-12.13%)	11.00% <0.00%> (ø%)
...va/com/github/davidmoten/rx/util/MapWithIndex.java	81.81% <0.00%> (-4.55%)	3.00% <0.00%> (ø%)
...en/rx/internal/operators/OperatorBufferToFile.java	82.17% <0.00%> (-2.33%)	4.00% <0.00%> (ø%)
...oten/rx/internal/operators/FileBasedSPSCQueue.java	72.99% <0.00%> (-2.19%)	12.00% <0.00%> (ø%)
...dmoten/rx/internal/operators/RollingSPSCQueue.java	50.00% <0.00%> (-1.82%)	18.00% <0.00%> (-1.00%)
...ub/davidmoten/rx/buffertofile/DataSerializers.java	53.84% <0.00%> (-1.06%)	3.00% <0.00%> (ø%)
...om/github/davidmoten/rx/testing/TestingHelper.java	77.77% <0.00%> (-0.08%)	27.00% <0.00%> (ø%)
src/main/java/com/github/davidmoten/rx/Jaxws.java	0.00% <0.00%> (ø)	0.00% <0.00%> (ø%)
... and 6 more

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