Faster not to compress intermediate bam2fq BAM file when sorting.

Using default compressed BAM as the intermediate file is slow:

    $ time samtools sort -n -O bam -T /tmp/TEMP_SORT /tmp/sample.bam | samtools bam2fq -s /tmp/singles.fastq - > /tmp/pairs.fastq
    [M::main_bam2fq] discarded 0 singletons
    [M::main_bam2fq] processed 116374 reads
    real         0m4.820s
    user         0m5.184s
    sys          0m0.189s

Using uncompressed BAM (the old samtools view -u option) faster,
as no need to gzip/ungzip the data being piped:

    $ time samtools sort -l 0 -n -O bam -T /tmp/TEMP_SORT /tmp/sample.bam | samtools bam2fq -s /tmp/singles.fastq - > /tmp/pairs.fastq
    [M::main_bam2fq] discarded 0 singletons
    [M::main_bam2fq] processed 116374 reads
    real         0m1.497s
    user         0m1.605s
    sys          0m0.245s

For reference, using SAM as the intermdediate format is faster than
default compressed BAM, but not quite as quick as uncompressed BAM:

    $ time samtools sort -n -O sam -T /tmp/TEMP_SORT /tmp/sample.bam | samtools bam2fq -s /tmp/singles.fastq - > /tmp/pairs.fastq
    [M::main_bam2fq] discarded 0 singletons
    [M::main_bam2fq] processed 116374 reads
    real         0m1.998s
    user         0m2.110s
    sys          0m0.249s

Times shown each best of three consecutive runs on the same server.