Nothing in ssb-server fetches blobs mentioned in replicated messages;
that was happening via ssb-blobs sympathy, which is the same mechanism
that let strangers fill the disk. This replaces it with explicit wants
scoped to feeds we actually replicate.
want() has no expiry and the want map is broadcast to every peer on
connect, so a standing want fires whenever a long-offline peer returns.
sbot keeps that map in memory, so this process ties its lifetime to the
connection and re-arms the backfill on every restart.
Verified: removing a referenced blob makes it report
already_have=266 newly_wanted=1 and issue the want.
