Add module event for repl-diskless-load swapdb

[oranagra]

When a replica uses the diskless-load swapdb approach, it backs up the old database,
then attempts to load a new one, and in case of failure, it restores the backup.

this means that modules with global out of keyspace data, must have an option to
subscribe to events and backup/restore/discard their global data too.

[oranagra]

@MeirShpilraien please let me know what you think.
do you see any reason to add tests for this? seems kinda pointless to me.

@redis/core-team please approve the interfaces or comment.

P.s. personally i don't like the "swapdb" option of repl-diskless-load that much.
it has possibility to trigger the kernel's OOM killer instead of actually restore a backup.
Personally i would have wanted to add an option to "always" use repl-diskless-load (taking the risk that a master failure at this stage will lead to complete data loss), this option doesn't exist today.
But anyway, this "experimental" feature exists (the entire repl-diskless-load, not just the swapdb option), so we probably need to let modules be compatible with it.
if some day we'll trim it, we can deprecate the interface, and never call the events.

[MeirShpilraien]

@oranagra small comment, other than this LGTM.