Query endpoint (Request handler path) shouldn't be hardcoded

[matslindh]

The (wrongfully, since everything technically is part of the same servlet inside the Java container) named constant SEARCH_SERVLET shouldn't really be a constant, as there might be multiple request handlers defined for a Solr collection or Solr core. These request handlers may define different default configuration options on the server side, and a user may want to use a different request handler.

Since the value is hardcoded in the library now, and the reference is resolved through self:: (and not static::, this value can't really be changed without matching 'select' in the _constructUrl path and modifying the servlet value for the container. This should probably be an optional part of the constructor or at least a setRequestHandler method.