The daemon's two-minute refresh compared the store's linked account
names with the names it booted with. A secondary that was named but did
not come up - no token at bring-up, a token the homeserver rejected
(M_UNKNOWN_TOKEN), or a homeserver still down after the ~52 s boot
retries - kept its name in the set, so nothing ever noticed it again:
it stayed down until the daemon restarted for some other reason, and
an operator re-linking a rejected account changed no name.
Each such secondary is now tried again on the same two-minute period,
one per tick, after one refresh and then doubling waits capped at
sixteen minutes. One that comes up joins the registry (now behind a
lock) and its sync loop joins the running set; the primary and the
other accounts are not restarted. The store is still only read.
The startup todo wipe moves out of bring_up_account to run once before
any sync loop: run per account, a late secondary would have cleared the
todos the running accounts' sweeps had already deduped.
Part of #5017 (slice M).
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