SequencesLoader: add readahead in the fastq loader

Link: https://github.com/sebhtml/ray/issues/137
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
Reviewed-by: Élénie Godzaridis <telomerika@gmail.com>
SequencesLoader: use a buffer of4M for fastq.gz files too

The fastq.gz code is also used for fasta.gz files.

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>