s3 sink to allow append to existing file

[chaporgin]

Currently S3 sink creates a new file each time it flushes the buffer. It would be nice if it could append data to an existing file.

[bruceg]

Unfortunately, the AWS S3 API only provides for writing whole files. There is no easy way of appending to an existing object. We do have an issue surrounding letting the S3 sink be more useful at creating large objects: #3829