* s3: auto-enforce bucket quota read-only both ways
Quota read-only only ever flipped when an admin re-ran
s3.bucket.quota.enforce, so a bucket that went over quota stayed
read-only forever even after usage dropped back under.
Fold enforcement into the per-minute, leader-locked bucket-size loop
the s3 gateway already runs for metrics: it now flips each bucket's
read-only flag to match its quota in both directions, rewriting
filer.conf only when a flag actually changes. The set/clear decision
lives in one shared FilerConf.ApplyBucketQuotaReadOnly helper so the
shell command and the gateway can't drift.
* only manage read-only when a quota is set, never clobber manual locks
* trim comments
Chris Lu
8c60408bfb