an id that isn't unique
A graph that failed after it was registered stayed in the map of this
server: it was answered for while it reported failed, and this server
never opened it again, so the failure was final. Take it back out and
close it, so that opening it can be tried again and the report of the
attempt that succeeds replaces the one that didn't.
Leave a server whose id can't be derived out of the readiness of its
graphs. The rest server url is the same for every server started from
one image, so the servers that fall back to it are one id to the
cluster, and one of them reporting ready used to answer for all of
them. Reporting the servers as unknown instead keeps their graphs below
ready, which is what the rest of this API does when it can't tell.
Relates to #3137
9faf6d572a