Bug 6884:
- a new subroutine AsyncLoop::bgsend_and_start_lookup() implements a
  commonly used idiom of calling bgsend(), followed by start_lookup(),
  while implementing a mechanism for grouping equal DNS queries,
  simplifying several other code sections;
- rewrite URIDNSBL.pm to put the new mechanism to good use,
  fixing the particular Bug 6884 and test 5 in t/uribl.t;
- fix the t/uribl.t, moving X_URIBL_FULL_NS from %patterns
  to %anti_patterns;
- add a dns_query_restriction configuration option;
- some cosmetic edits, like renaming some variables for consistency.

PR:		6884
Submitted by:	Mark.Martinec@ijs.si


git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1434360 13f79535-47bb-0310-9956-ffa450edef68