fix(rtable): Potential memory leak in rte replace

If a route table entry was overlaid during the loading of
a new table there was a potential for leaking. Added unit
tests to better verify results of symtab cloning.

Change-Id: I6afecf52e9a48b36da3a06fa01867956a1a52261
Signed-off-by: E. Scott Daniels <daniels@research.att.com>