ike-sa-manager: Use transient hasher for IKE_SA_INIT hash calculation

To check if a received IKE_SA_INIT request is a new request or a
retransmit, charon maintains hashes of the pending IKE_SA_INIT
exchanges.

However, the hash calculation is not reentrant because a single hasher
is used for the whole IKE SA manager. It leads to bogus calculations
under high load and hence dropped messages on responder
(IkeInInvalidSpi incremented).

Don't share a single hasher in the IKE SA manager, create a transient
one whenever a message must be hashed.

Signed-off-by: Christophe Gouault <christophe.gouault@6wind.com>