BAMSplitGuesser: treatment of Picard exceptions

Some newer BAM files from 1000Genomes seem to cause issues for the
BAMSplitGuesser. This commit should fix the problem that caused
an incorrect offset for the start of a SAMRecord within the
uncompressed BGZF block. Also a debug mode that lists the split
point coordinates was added. For reference, one affected file was:

ftp://ftp.1000genomes.ebi.ac.uk/vol1/ftp/data/NA12878/high_coverage_alignment/NA12878.mapped.ILLUMINA.bwa.CEU.high_coverage_pcr_free.20130906.bam
Merge remote-tracking branch: upstream/master