* filer_pb: add MkdirExclusive
A taken name leaves the callback as a result, not an error, so filer
failover does not count a lost race as a filer failure.
* s3tables: create the entry with its attributes in one exclusive create
CreateTableBucket and CreateNamespace checked that the name was free and
then wrote the entry in several calls, so a second create in the window
replaced the first caller's entry including its owner. The create now
carries the marker, metadata and tags in one O_EXCL CreateEntry that
answers ErrEntryAlreadyExists with the same 409.
* s3: PutBucket uses an exclusive mkdir
The same window let a plain PutBucket replace a table bucket created
concurrently. The already-exists branch now answers from the entry itself,
since the registry just cached the name as absent, and the admin
auto-create path is exclusive too.
* s3: read the winning entry when a create loses the race
The concurrent-create branch called existingBucketError on a name the
registry had just cached as absent, answering NoSuchBucket, and the
auto-create fallback could assign an S3 owner to a table bucket it met
there. The negative cache is cleared before the existing-bucket error is
built, and a table bucket keeps its owner-less shape.
* s3: fail an auto-create that met a table bucket
A table bucket carries no S3 owner and must not gain one, and the upload
that auto-create ran for must not continue into it either.
* s3: fail an auto-create when the winning entry cannot be read
An unreadable existing entry fell through to success, so the write
continued into an unvalidated entry, possibly a table bucket.
