fix score boosting issues

[mschoch]

While investigating the behavior of several queries using
the query boost capability a few issues were found.

First, an optimization in the boolean query, would replace
the boolean searcher with internal "should" searcher, in
the case where it was the only one (no "must" or "must not").
This optimization is valid, but as implemented today, it looses
track of the users boost. As this seems to be a minor
optimization, it is temporarily disabled to allow for correct
boosting behavior.

Second, the boolean searcher, which by default will use a
composite scorer (sum constituent scores), was ignoring the
user requested boost value. A new boosting variant of the
composite scorer is introduced. This variant first sums
the scores, then multiplies by the boost.

Third, the numeric range searcher was defaulting to a constant
score of 1.0, and it too ignored the requested boost value. The
code has been updated to return a constant score equal to the
boost (1.0 * boost).

[mschoch]

NOTE: while investigating these issues, I have found a limitation in the way we allow for custom scoring. Due to the API design, when using custom scorers, it is not possible for them know (and therefore use) a user specified boost. At this time it's not clear if this belongs internal to the scoring, or if perhaps all boosting should be external to scoring. This PR does not attempt to address this realization at all, rather it focuses on improving behavior with the existing API limitations.