The unified scope is rebuilt on every connection while a browser restores
its logins, on every background push, and on every unified browse,
load-more and search - and each build asked every login for its folders
again, one login after the other. With ten logins that was ten full
Mailbox/get per build and well over a hundred in the first seconds, and the
slowest build waited on the sum of every login's round trip.
Fetch the logins in parallel, and fetch each login's list once: a build
shares a fetch that is already in flight and reuses a finished one until
that login reports a change - the push handlers invalidate the login they
hear from, so a background push now costs one Mailbox/get instead of one
per login. A 60 s limit backstops a change whose push was lost.
A reused list is never written back into `accountMailboxes`: optimistic
updates land there directly, and publishing an older list over them would
snap a just-changed counter back.
lucamzanon
a578150c2d