Bug fix: memory used by GRS records wasn't freed.

Bug fix: memory used by GRS records wasn't freed.
Rewrote some of the error handling code - the connection is always
closed before failback is called.
If failback is defined the send APDU methods (init, search, ...) will
return OK but invoke failback (as is the case if the write operation
fails).
Bug fix: ref_count in assoc object could grow if fraction of PDU was
read.