Performance enhancements

Multi-threaded scheme was slower than single-threaded. There were multiple causes:
-- a bug in atom-table was causing link outgoing sets to not get resolved.
-- thus, resolution was being done at run-time,
-- which bottle-necked on the atom table get-handle mutex

Aside from this, some other, "minor" cleanups: 
-- store handles directly in scheme, so as to avoid lookup,
-- avoid old atomspace silliness in the pattern-matcher