Use deferred and callbacks instead of `:sync t` for tkf requests which
is known to have issues. Query server attributes once on
notebooklist-open to avoid sequencing issue #176 (but allow Resync).
Under curl backend, a second request for the same "key" as a pending
request will abort the latter, which has resulted in a clobbered
curl-cookie-jar file, so merely warn and don't abort.
Fix#176
Passing "default" to `ein:get-kernelspec' would cause it to return a string
naming the actual kernelspec, which just won't do. So we look for that case and
then look for the real kernelspec.
