A config-file reload (SIGHUP) routed through MergeS3ApiConfiguration,
which skips identities marked static so dynamic admin/filer updates can't
clobber them. That also blocked the config file itself from updating its
own identities, so editing a secretKey and reloading had no effect.
Thread a fromStaticFile flag from the file-load path into the merge: the
authoritative file overwrites its static identities (and reapplies service
accounts under them), while dynamic updates still leave them immutable.
Mark the rebuilt identities static in the merge so a concurrent
RemoveIdentity never observes them as removable mid-reload.
Chris Lu
089acfbf36