Remove DEFAULT_LAZY_LOAD parameter
Have padding bases get treated as a zero length gap

Add unit test for same

IGV-1971
Reuse buffer contents when possible in IGVSeekableBufferedStream
Add gff unit test
gff
Overhaul of GFF parsing to (1) create less intermediate objects, and (2) properly account for 3' and 5' UTR records.  Still to do -- overhaul treatment of popup text for features with children (mRNAs with exons & cds, etc).