charon-tkm: Call esa_reset() when the inbound SA is deleted

After a rekeying the outbound SA and policy is deleted immediately, however,
the inbound SA is not removed until a few seconds later, so delayed packets
can still be processed.

This adds a flag to get_esa_id() that specifies the location of the
given SPI.

testing/tests/tkm/xfrmproxy-rekey/hosts/moon/etc/strongswan.conf

@@ -1,6 +1,8 @@
 # /etc/strongswan.conf - strongSwan configuration file
 
 charon-tkm {
+  # remove rekeyed inbound SA a bit quicker for the test scenario
+  delete_rekeyed_delay = 2
   dh_mapping {
     15 = 1
     16 = 2