The start session method is reworked to pass a response back to the
client.
If the method was successful, a session list entry is created. This
is different to the provious behaviour, where we created the
session list entry unconditionally.
This new arrangement means that we need a different way to avoid
a race condition where two users may try to create a session at the
same time, and end up with the same display. We do this by keeping
track of newly allocated displays as part of the SCP connections. When
we allocate a display, the SCP connection displays are also taken
into account.
matt335672