Unify and cleanup usage of hash_set/unset_mem by various tunnels

Move the functions hash_set_key_copy() and hash_unset_key_free()
which are dupilicated in various tunnel support code modules to
hash.h as hash_set_mem_alloc() and hash_unset_mem_free() to be
used by all.

Change-Id: I40723cabe29072ab7feb1804c221f28606d8e4fe
Signed-off-by: John Lo <loj@cisco.com>