Optimize advanced search by relationship with target group for reciprocal relationship types

On many sites, this is not an issue, but the more "spouse" relationships a site has, the more likely it happens (and not just spouse; this applies for "partner of" and any other reciprocally named relationship type).

Steps to reproduce:

1. Start with a site having:
   1. Several thousand active relationships of a reciprocal type (where the name for both related positions is the same – e.g., "spouse of" <=> "spouse of"; "sibling of" <=> "sibling of"; "partner of" <=> "partner of"). This recipe uses "Spouse of".
   2. At least one group (smart group or static group).
2. Perform an Advanced Search with these criteria:
   1. Relationships > Relationship Type: "Spouse of"
   2. Relationships > Target Contact(s) in Group: [any one or more groups]
3. Submit the search form and observe a very long page load; depending on the number of relationships, server configuration, and server resources, this search may end in a WSOD, or take dozens of minutes to complete.

Proposed solution:

• Expand existing code constructs, already in use for reciprocal relationship types in Relationships > Relationship Type when Relationships > Target Contact(s) in Group is undefined. That is, create a temporary table using UNION to flatten reciprocal relationships and avoid using OR in joins (because OR in a join will prevent MySQL from using indexes in that join).