Array-based ring implementation.
PushErr/PushLog() for a Bucket.

Expanded (and renamed) the statsLock to also help cover the errs and
logs rings.
Invoke PushErr/PushLog during map func.
Bucket.Errs()/Logs() return the last errors/logs.
REST /_api/buckets/BUCKET/errs & logs.
TestRestGetBucketErrs/LogsEmpty().
Move FileService.OpenFile() to file_server.go.

It lives nearer to all the other FileService methods.