common: add destructor to intrusive_lru

common: add destructor to intrusive_lru

values created during inserts that are
unreferenced are not deleted because evict()
does not delete values unless set.size() > lru_target_size.

For example, this condition is not met if the number
of entries in the intrusive_lru are less than or
equal to lru_target_size at the time of the
intrusive_lru's destruction.

Memory leaks due to the above scenario can be
reproduced if this commit is removed and
unittest_intrusive_lru is run under valgrind.

ex:

valgrind --tool=memcheck --leak-check=yes ./bin/unittest_intrusive_lru

Signed-off-by: Ali Maredia <amaredia@redhat.com>