Improve data retrieval performance for large data chunks

[maciejlach]

Performance of the QReader.Read() doesn't scale linearly with size of data to be retrieved.

Data sample:

gen: {[x] sample::([] ti:09:30:00.0 +x?06:00:00.0 ;a:100 +x?100f ; ap:x?50;b:101 +x?100f ; ab:x?50; id: x?10)}

Sample results:

sample size	qSharp [ms]	c.cs [ms]
50000	47	47
75000	62	78
100000	140	172
500000	483	390
1000000	1591	687
1500000	3385	1030

[maciejlach]

Sample retrieval results after applying the fix:

sample size	qSharp [ms]	c.cs [ms]
50000	33	32
75000	47	52
100000	58	64
500000	259	320
1000000	504	638
1500000	782	931