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 characters.

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
Fix IndexOutOfBounds error when deserializing max_by/min_by aggregation state

It happens because deserialize mistakenly reads the length from the first
position in the block regardless of which position is being deserialized.

If the length of the first position is longer than the length of the current
position, the code tries to read beyond the end of the buffer.
Improve error message when types don't match in UNION query
Fix broken analyzer test

Forgot to update the tests in df99f33ab82b20f6afc362c7eb1d165b30ef8441
Add documentation for min_by
Update reference to floatingdecimal in docs