Change numeric data types to use SORTED_NUMERIC docvalues type
instead of a custom encoding in BINARY.

In low level benchmarks this is 2x to 5x faster: its also optimized
for the common case where fields actually only contain at most one
value for each document.

Additionally SORTED_NUMERIC doesn't lose values if they appear more
than once, so mathematical computations such as averages are correct.

Closes #6967