rgw_cksum: multipart upload checksums

rgw_cksum: multipart upload checksums

includes commits:

* fixes init-multipart header return
* introduce checksum to SAL MultipartPart interface
* thread optional checksum through DataProcessor
* code complete multipart checksum verify
* fix formatter
* fix ckecksum format for multipart objects in GET/HEAD ops
* always return parts_count from ReadOp::prepare() if applicable
      This behavior is used when returning the checksum of a multipart
      upload object.
* tweak conditional multipart_parts_count
* add checksum output to ListMultipart
* fix nil-return from GetHeaderCksumResult
* re-arm truncated if re-entering list-parts
* complete-multipart w/list-parts
* validate supplied checksum in CompleteMultipart
* verify checksum type against initial checksum algorithm
* rgw_op: suppress more x-amz headers
* final fixes and cleanups
* remove unused t0

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>