If someone requests a compose that is already being produced or has been produced, return the ID of that compose instead of a creating a new one.

Anytime the compose is re-requested, then the EOL should be extended a little further in the future --
extend it, as if it were a new compose... but never shorten the EOL (there is some complex time logic to be written here).

Note that @mikeb left a note in the focus doc pointing to a koji API call we'll want to use here that can help us determine if the contents of the tag have or have not changed since the last compose.