Fix the missing message detection in the client subscription
Fix the missing message detection in the client subscription

More than one consecutive keepalive message and the first notification
message after a keepalive message share the same sequence number.
There should be no warning for these cases.

jpfr/open62541,StalderT/open62541,StalderT/open62541,jpfr/open62541,JGrothoff/open62541,open62541/open62541,JGrothoff/open62541,open62541/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541,open62541/open62541,StalderT/open62541,jpfr/open62541,jpfr/open62541,JGrothoff/open62541