Allocate session UID with count increment

Otherwise, the UST session UID gets assigned only when the first UST
channel is created for a session, causing IDs to be used for many
sessions (the count of the number of sessions allocated so far) if the
session creation / channel creation is not performed in a usual
sequence.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>