Support loading of CSI from URLs/streams.

[cmnbroad]

Fixes https://github.com/samtools/htsjdk/issues/1507.

[cmnbroad]

@lbergelson Yeah the comments were all good improvements. Thanks for reviewing...

[codecov-commenter]

Codecov Report

Merging #1595 (c094474) into master (b5af659) will decrease coverage by 0.041%. The diff coverage is 75.000%.

@@               Coverage Diff               @@
##              master     #1595       +/-   ##
===============================================
- Coverage     69.839%   69.798%   -0.041%     
- Complexity      9647      9660       +13     
===============================================
  Files            702       703        +1     
  Lines          37638     37680       +42     
  Branches        6113      6097       -16     
===============================================
+ Hits           26286     26300       +14     
- Misses          8903      8920       +17     
- Partials        2449      2460       +11     

Impacted Files	Coverage Δ
src/main/java/htsjdk/samtools/BAMFileReader.java	68.661% <57.143%> (-0.840%)	:arrow_down:
src/main/java/htsjdk/samtools/SamIndexes.java	75.714% <83.333%> (+2.637%)	:arrow_up:
src/main/java/htsjdk/samtools/CSIIndex.java	71.622% <100.000%> (ø)
...ava/htsjdk/samtools/CompressedIndexFileBuffer.java	58.974% <100.000%> (+4.689%)	:arrow_up:
...n/java/htsjdk/samtools/IndexFileBufferFactory.java	60.000% <100.000%> (+26.667%)	:arrow_up:
...dk/samtools/cram/structure/SubstitutionMatrix.java	74.016% <0.000%> (-14.173%)	:arrow_down:
...sjdk/samtools/util/htsget/HtsgetErrorResponse.java	73.333% <0.000%> (-6.667%)	:arrow_down:
...tools/cram/encoding/core/GammaIntegerEncoding.java	86.667% <0.000%> (-6.667%)	:arrow_down:
.../main/java/htsjdk/samtools/AsyncSAMFileWriter.java	33.333% <0.000%> (-5.128%)	:arrow_down:
...mtools/cram/encoding/core/BetaIntegerEncoding.java	91.304% <0.000%> (-4.348%)	:arrow_down:
... and 23 more