Use BINARY doc values instead of SORTED_SET doc values to store numeric data.

Although SORTED_SET doc values make things like terms aggregations very fast
thanks to the use of ordinals, ordinals are usually not that useful on numeric
data. We are more interested in the values themselves in order to be able to
compute sums, averages, etc. on these values. However, SORTED_SET is quite slow
at accessing values, so BINARY doc values are better suited at storing numeric
data.

floats and doubles are encoded without compression with little-endian byte order
(so that it may be optimizable through sun.misc.Unsafe in the future given that
most computers nowadays use the little-endian byte order) and byte, short, int,
and long are encoded using vLong encoding: they first encode the minimum value
using zig-zag encoding (so that negative values become positive) and then deltas
between successive values.

Close #3993