Finds an OperationContext for an incoming message. An incoming message can be of two states.
1)This is a new incoming message of a given MEP. 2)This message is a part of an MEP which has
already begun.
The method is special cased for the two MEPs
#IN_ONLY #IN_OUT
for two reasons. First reason is the wide usage and the second being that the need for the
MEPContext to be saved for further incoming messages.
In the event that MEP of this operation is different from the two MEPs defaulted above the
decision of creating a new or this message relates to a MEP which already in business is
decided by looking at the WSA Relates TO of the incoming message.