rgw/beast: disable keepalive for responses before 100 Continue

rgw/beast: disable keepalive for responses before 100 Continue

resolves a long-standing category of delays/errors from boto3 for
requests like s3:PutObject that contain a request body and use the
`Expect: 100-continue` request header. if the server rejects such
requests based on their headers alone, it will respond with an http
status like `HTTP/1.1 403 Forbidden` without first sending the
`HTTP/1.1 100 Continue` status

when http keepalive is enabled, this situation creates an ambiguity for
the server when trying to read the next request from that connection -
subsequent bytes may either correspond to the current request's body or
the next request's headers

to resolve this ambiguity, disable http keepalive by sending response
header `Connection: close` instead of `Connection: keep-alive`. this
instructs the client and server to close this connection instead of
reusing it for subsequent requests

Fixes: https://tracker.ceph.com/issues/42208
Signed-off-by: Casey Bodley <cbodley@redhat.com>