Stop writing fileblob's .attrs sidecar - #328

fileblob stores blob metadata in an ".attrs" file per object and rewrites
it with os.Create, truncating in place outside the atomic rename that
protects the blob. A read overlapping a write decodes a partial file and
fails with "opening reader: EOF", served as a 502. One writer against
four readers on a single key failed 408 of 2000 reads. cacheMetadataBlob
is most exposed to it, rewriting a key on every refresh while readers
are served from it.
Nothing in the proxy reads what the sidecar holds. gocloud.dev/blob is
imported only by internal/storage, Store sets no ContentType, and
Attributes is used only for Size, which comes from os.Stat. A missing
sidecar already defaults cleanly, so "metadata=skip" removes the hazard
rather than locking around it, and saves a write per store.

metadata=skip stops fileblob rewriting sidecars but does not delete ones
already on disk, so a sidecar left partial by an interrupted write now
fails every read of its key for good. Before, a later store repaired it
by rewriting.
Store therefore removes the sidecar for the key it writes. Removal is
atomic where the rewrite was not, so a concurrent reader gets the whole
old file or nothing. Delete already removes sidecars, so the two paths
drain a cache between them.
Deriving that path is necessary because fileblob's key escaping is
unexported. It is the identity for a plain key and parts from one only
for keys that are not valid local paths, which is what filepath.Localize
rejects. That also keeps the removal inside the cache directory: without
it a key holding ".." resolves outside.
The clearing test runs one key per storage path the proxy builds, seeded
through a bucket that still writes sidecars so the path under test is
fileblob's own.