Improved handling of create API call

[Ithanil]

During work on #1049, it turned out the handling of the create API call could use some improvements. These are especially useful in the context of the mentioned PR, but are independent of it.

Description

Changes:

1. In bigbluebutton_api_controller.rb, begin with checking whether the meeting is already running and only when nothing is found, continue with find_available servers and meeting find_or_create_with_server. When an existing meeting is found, use the returned meeting information directly, instead of the same information that would have been returned from find_or_create_with_server.
   This is mostly an improvement in terms of logic (e.g. allowing for 2) ) and also a performance improvement for the create call (which frontends may repeat before every single join call). This may be slightly more relevant in the context of Support for tagged servers (ready for review) #1049, where the find_available routine becomes slightly more expensive.
2. Only increment server_load if no meeting was found in 1), i.e. when a new meeting is actually created. To compensate, increment server_load on every join call. I think the previous implementation did rely on frontends always pairing join calls with a create call, to adjust the preliminary load values. In any case, the poller will take care of proper load values after a while.

Testing Steps

Existing automated tests + used in actual production deployment, in combination with #1049 .

Notes

• A notable change in behavior is that before this patch, a create call would always fail if Server.find_available threw an error, even if the meeting would already exist in the database. With this patch it would indeed try to forward the create call to the corresponding server, even if the said server is not registered as available anymore. Before this patch it would do exactly the same in the end, but only if any other server was found in find_available, from which it then would fall back to the registered server for the meeting.

[ffdixon]

Thanks @Ithanil for the improvements. We're working to schedule time to review.

[ffdixon]

Thanks again @Ithanil, will have feedback for you soon.