Disable use of virtual hosting in aws-sdk-cpp

Recently aws-sdk-cpp quietly switched to using S3 virtual host URIs
(https://github.com/aws/aws-sdk-cpp/commit/69d9c53882), i.e. it sends
requests to http://<bucket>.<region>.s3.amazonaws.com rather than
http://<region>.s3.amazonaws.com/<bucket>. However this interacts
badly with curl connection reuse. For example, if we do the following:

1) Check whether a bucket exists using GetBucketLocation.
2) If it doesn't, create it using CreateBucket.
3) Do operations on the bucket.

then 3) will fail for a minute or so with a NoSuchBucket exception,
presumably because the server being hit is a fallback for cases when
buckets don't exist.

Disabling the use of virtual hosts ensures that 3) succeeds
immediately. (I don't know what S3's consistency guarantees are for
bucket creation, but in practice buckets appear to be available
immediately.)

src/libstore/s3-binary-cache-store.cc

@@ -79,7 +79,7 @@ static void initAWS()
 
 S3Helper::S3Helper(const string & region)
     : config(makeConfig(region))
-    , client(make_ref<Aws::S3::S3Client>(*config))
+    , client(make_ref<Aws::S3::S3Client>(*config, true, false))
 {
 }