Revision 149922

Revised internal search APIs.  Changes include:

  a. Queries are no longer modified during a search.  This makes
  it possible, e.g., to reuse the same query instance with
  multiple indexes from multiple threads.

  b. Term-expanding queries (e.g. PrefixQuery, WildcardQuery,
  etc.)  now work correctly with MultiSearcher, fixing bugs 12619
  and 12667.

  c. Boosting BooleanQuery's now works, and is supported by the
  query parser (problem reported by Lee Mallabone).  Thus a query
  like "(+foo +bar)^2 +baz" is now supported and equivalent to
  "(+foo^2 +bar^2) +baz".

  d. New method: Query.rewrite(IndexReader).  This permits a
  query to re-write itself as an alternate, more primitive query.
  Most of the term-expanding query classes (PrefixQuery,
  WildcardQuery, etc.) are now implemented using this method.

  e. New method: Searchable.explain(Query q, int doc).  This
  returns an Explanation instance that describes how a particular
  document is scored against a query.  An explanation can be
  displayed as either plain text, with the toString() method, or
  as HTML, with the toHtml() method.  Note that computing an
  explanation is as expensive as executing the query over the
  entire index.  This is intended to be used in developing
  Similarity implementations, and, for good performance, should
  not be displayed with every hit.

  f. Scorer and Weight are public, not package protected.  It now
  possible for someone to write a Scorer implementation that is
  not in the org.apache.lucene.search package.  This is still
  fairly advanced programming, and I don't expect anyone to do
  this anytime soon, but at least now it is possible.

Caution: These are extensive changes and they have not yet been
tested extensively.  Bug reports are appreciated.