charon-tkm: Add get_dst_host getter to SAD

This function returns the destination host of an SAD entry for given
reqid, spi and protocol arguments or NULL if not found.
charon-tkm: Use get_dst_host getter in EES callback

Use the new get_dst_host getter to retrieve the destination host from
the SAD using the reqid, spi and protocol values received from the
xfrm-proxy.
testing: Assert ees acquire messages in xfrmproxy tests
testing: Add tkm xfrmproxy-expire test

This test asserts that the handling of XFRM expire messages from the
kernel are handled correctly by the xfrm-proxy and the Esa Event Service
(EES) in charon-tkm.