12190: Don't test block existence before first write.

12190: Don't test block existence before first write.

"Amazon S3 provides read-after-write consistency for PUTS of new
objects in your S3 bucket in all regions with one caveat. The caveat
is that if you make a HEAD or GET request to the key name (to find if
the object exists) before creating the object, Amazon S3 provides
eventual consistency for read-after-write."

-- http://docs.aws.amazon.com/AmazonS3/latest/dev/Introduction.html#ConsistencyModel

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>