Reader for paired ends that are spread across two FASTQ files.
The number of records read from each file is synchronized, but the number is not known until
after reading the first time, when we determine how many reads can fit into the fixed size
buffer. While the buffer may grow later for each RecordSet later (if later reads are
longer), this is a good approximation given the buffer size.
Stores two record sets with the same number of records for paired end reads. Each record set
is used to read from the corresponding reader, one per end of a pair.