remove redundant calls to update_timestamp() and sign()

S3Request.__init__() was explicitly calling both update_timestamp()
and sign(), which were both then being called again in get_triplet()
before issuing the actual request.  This made the values calculcated
under __init__() redundant.

Worse, now with signature v4 code, sign() calculates a sha256() over
the whole body of the request, which could be a very large file.  So
calling sign() an extra time uses extra CPU and I/O banwidth that
isn't necessary.

Patch removes the extra calls from S3Request.__init__().
remove extraneous call to checksum_sha256()

In object_put() we were calculating the checksum in an argument to
call create_request().  But we were calculating the checksum a second
time in the call to send_file().  For large files, this would invoke
file I/O for a second time, and in any case wasted CPU.

So, we remove the call to checksum_sha256() in the invocation of
create_request() in object_put().