stats: Add support for exponential stats group-by fxn

stats: Add support for exponential stats group-by fxn

One can specify the quantization parameters in the config file as:

    <field>:exponential:<min mag>:<max mag>:<base>

Currently, only base 2 and base 10 are supported.

For example:

    group_by = bytes_out:exponential:1:4:10

Which will quantize the bytes_out values into the buckets: (-inf, 10],
(10,100], (100,1000], (1000, 10000], (10000, +inf).