This adds a parameter "events" to the /rest/events endpoint. It should
be a comma separated list of the events the consumer is interested in.
When not given it defaults to the current set of events, so it's
backwards compatible.
The API service then manages subscriptions, creating them as required
for each requested event mask. Old subscriptions are not "garbage
collected" - it's assumed that in normal usage the set of event
subscriptions will be small enough. Possibly lower than before, as we
will not set up the disk event subscription unless it's actually used.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4092
d48e46a29c