Fix: consumer: 64-bit index for relayd rather than 32-bit (v2)

Fix: consumer: 64-bit index for relayd rather than 32-bit (v2)

Relayd "unique" ids wrap every 32-bit, and in some cases, negative
values are considered as error.

Change this to make the error value specifically -1ULL, use a direct
comparison (since we use an unsigned 64-bit integer, comparison with 0
becomes incorrect).

Since we now use a 64-bit ID, it is assumed to _never_ wrap-around
(remember, value -1ULL is an _error_). Therefore,
consumer_add_relayd_socket() can become much more strict than it was:
instead of accepting re-use of net_seq_idx, we can now assert that upon
LTTNG_STREAM_CONTROL socket, we have indeed allocated a relayd object,
and upon LTTNG_STREAM_DATA, we have found a relayd object.

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