Fix: relayd vs consumerd compatibility

Fix: relayd vs consumerd compatibility

relay and consumerd 2.7 and 2.8 are expected to negociate compatibility
with the lowest common minor version.

If a consumer daemon 2.8 interacts with a relayd 2.7, it needs to send
the index fields for ctf index 1.0. Same if a relayd 2.8 interacts with
a consumer daemon 2.7: relayd should expect ctf index 1.0 fields, and
generate a ctf index 1.0 index file layout.

If both relayd and consumerd versions are 2.8+, then we can send the ctf
index 1.1 fields over the protocol, and store them in the index files.

Whenever the relayd live viewer server opens and reads an index file,
it needs to use the file's header to figure out the index "element"
size.

[ Should be applied to master, stable-2.9, stable-2.8. ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>