Closes #3767: Add batch string read

[bmcdonald3]

Currently, Parquet strings are read and processed one at a time, but we'd like to read them in batches to improve performance. Here are some results from a Cray XC with Lustre filesystem:

Batch read:

test	sec
single-file	4.539
fixed-single	0.779
scaled-five	2.06
fixed-scaled-five	0.398
five	10.046
fixed-five	1.507
scaled-ten	1.086
fixed-scaled-ten	0.261
ten	10.906
fixed-ten	2.643

Old:

test	sec
single-file	7.907
fixed-single	4.026
scaled-five	4.021
fixed-scaled-five	1.754
five	17.076
fixed-five	4.997
scaled-ten	1.782
fixed-scaled-ten	0.978
ten	17.802
fixed-ten	9.499

Closes #3767

[stress-tess]

Looks great! thanks ben!!