Fix memory leaks revealed by valgrind and ensuing code inspection.

In the existing test suite valgrind revealed two memory leaks (DB_get
and DBC_set_range).  Code inspection revealed that there were many other
potential similar leaks (many on odd code error paths such as passing
something other than a DBTxn object for a txn= parameter or in the face
of an out of memory error).  The most common case that would cause a
leak was when using recno or queue format databases with integer keys,
sometimes only with an exception exit.