Use CFS in any case if index.compound_format is set to true

Lucenes MergePolicies support a noCFSRatio. This commit introduces
support for this ratio via `index.compound_format`. This setting
can parse a boolean value or a value in the interval [0..1] that
is equivalent to the noCFSRatio. The setting `1`, `1.0` and `true`
are equivalent as well as `0`, `0.0` and `false`.

Closes #3166