Prepare subnet selection speedup: auxiliary tables

There are at least two cases where an index can or should not be used/added to a multi index:

• when the corresponding data structure is not supported, e.g. patricia tree for IP prefixes
• when the default is very common: the multi index works but gives a lot of values for this default value so it doubles the insert/delete operation time
• when a direct index value change is convenient: this breaks the multi index so must not be done (instead the multi index must be updated which is a complex (to code) and slow (to execute) operation)

The solution is to add in parallel of a multi index soem auxiliary tables which provide a specific lookup. The auxiliary table is a base type with add, delete and change (which is in general delete followed by add).