Avoid using S3 multipart uploads for small files

From sonatype/nexus-public#39. It allows the S3 blob store to avoid using a multipart request when the upload would just use one chunk. Specifically, it will use a single putObject request instead of a sequence of IntiateMultipartUpload, UploadPart, and CompleteMultipartUpload. This cuts down on requests between NXRM and S3 and improves performance. Additionally, it fixes compatibility with the third-party S3 service NetApp StorageGrid, which enforces a minimum upload size for multipart.