Complete IUniLinks interface implementation

The implementation of single Trigger method can be useful to use specific optimizations for each case, using less amount of memory accesses. For example `GetOrCreate` method now executes `SearchOrDefault` method twice, directly and indirectly. When we use single Trigger method is it is easy to know the intend of user's request and do just necessary operations and no more.

Start by locating the IUniLinks interface and its Trigger, GetOrCreate, and SearchOrDefault members. Trace how GetOrCreate currently reaches SearchOrDefault directly and indirectly, then complete the single-Trigger implementation so each request performs only the necessary operations without duplicate searches.

Written by the indexing model from the issue text.