Temporarily disable string statistics in orc/drwf readers

The writer performs comparisons using java Strings to determine the minimum and maximum
values. This results in weird behaviors in the presence of surrogate pairs and special character (see http://icu-project.org/docs/papers/utf16_code_point_order.html)

For example, unicode codepoint 0x1D403 has the following representations:
- UTF-16: [0xD835, 0xDC03]
- UTF-8:  [0xF0, 0x9D, 0x90, 0x83]

while codepoint 0xFFFD (the replacement character) has the following representations:
- UTF-16: [0xFFFD]
- UTF-8:  [0xEF, 0xBF, 0xBD]

when comparisons between strings containing these characters are done with Java Strings (UTF-16),
0x1D403 < 0xFFFD, but when comparisons are done using raw codepoints or UTF-8, 0x1D403 > 0xFFFD