Improved matchall support.

* Infinite matchalls now supported, e.g.: "apple" []* "pear"
* Fixed parsing of matchalls to allow them to occur at beginning or end
as well, e.g.: []* "apple"
* Fixed bug when multiple consecutive matchalls occurred at the end of
a query.
* Refactored "get field length" functionality from SpansNot into
separate class (DocFieldLengthGetter) for reuse
* Fixed SpanQueryExpansion to check field length when expanding to
the right