Add the SIP callid as a key for persistence.

This allows multiple connections from the same IP address to be
differentiated on the basis of the callid.

When used in conjunction with the persistence mask, it allows connections
from different  IP addresses to be aggregated on the basis of the callid.

It is envisaged that a persistence mask of 0.0.0.0 will be a useful
setting.  That is, ignore the source IP address when checking for
persistence.

It is envisaged that this option will be used in conjunction with
one-packet scheduling.

This only works with UDP and cannot be made to work with TCP
within the current framework.
Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Julian Anastasov <ja@ssi.bg>