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."
Arvados-DCO-1.1-Signed-off-by: Tom Clegg <email@example.com>