Memoize specialized functions

[dpino]

Calling these functions several times without memoizing them can cause a TABOV (table overflow) exception.

I tested that in fact if calling BTLookupQueue.new() many times causes a TABOV (table overflow) exception. Example:

local inet_lookup_queue
for i=1,10000 do
   inet_lookup_queue = BTLookupQueue.new(map)
end

Memoizing these functions makes the snippet of code above work.