Some good ideas from @dickmao, the only difference here is that notebook
autosave is not defaulted to being turned off as this might produce some
unexpected behavior for existing users. What I am doing is decreasing the
default frequency to autosaving once every five minutes.
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
