The internal cache and munge parts of the global and attr handlers have been
factored out. This allows us to wrap the global and attr handlers and to
short-circuit them, yet still support pre/post cache and munge lookups.
Also added the StoreSession feature which dumps received raw data from
Net-SNMP to a text file in "qual_leaf = var" format. Enabled using
StoreSession param to new(), and filename optionally set with SessionDataFile.
