Bring etcd support for bucket DNS federation

- Supports centralized `config.json`
- Supports centralized `bucket` service records
  for client lookups
- implement a new proxy forwarder

docs/federation/lookup/Corefile.example

@@ -0,0 +1,11 @@
+. {
+  etcd churchofminio.com {
+      endpoint http://localhost:2379 http://localhost:4001
+      upstream /etc/resolv.conf
+  }
+  debug
+  prometheus
+  cache 160 churchofminio.com
+  loadbalance
+  proxy . /etc/resolv.conf
+}

docs/federation/lookup/README.md

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+# Federation
+There are primarily two types of federation
+
+- Bucket lookup from DNS
+- Bucket is shared across many clusters
+
+This document will explain in detail about how to configure Minio supporting `Bucket lookup`
+
+## Federation (Bucket Lookup)
+Bucket lookup federation requires two dependencies
+
+- etcd (for config, bucket SRV records)
+- coredns (for DNS management based on populated bucket SRV records)
+
+## Architecture
+
+![bucket-lookup](./bucket-lookup.png)
+
+### Run Multiple Clusters
+> cluster1
+```
+export MINIO_ETCD_ENDPOINTS="http://remote-etcd1:2379,http://remote-etcd2:4001"
+export MINIO_DOMAIN=domain.com
+export MINIO_DOMAIN_IP=44.35.2.1
+minio server http://rack{1...4}.host{1...4}.domain.com/mnt/export{1...32}
+```
+> cluster2
+```
+export MINIO_ETCD_ENDPOINTS="http://remote-etcd1:2379,http://remote-etcd2:4001"
+export MINIO_DOMAIN=domain.com
+export MINIO_DOMAIN_IP=44.35.2.2
+minio server http://rack{5...8}.host{5...8}.domain.com/mnt/export{1...32}
+```
+
+In this configuration you can see `MINIO_ETCD_ENDPOINTS` points to the etcd backend which manages Minio's
+`config.json` and bucket SRV records. `MINIO_DOMAIN` indicates the domain suffix for the bucket which
+will be used to resolve bucket from DNS. For example if you have a bucket such as `mybucket`, the
+client can use now `mybucket.domain.com` to directly resolve to the right cluster. `MINIO_DOMAIN_IP`
+points to the public IP address where each cluster might be accessible, this is unique per each cluster.
+
+NOTE: `mybucket` only exists on one cluster either `cluster1` or `cluster2` this is truly random and
+is decided by how `domain.com` gets resolved, if there is a round-robin DNS on `domain.com` then
+it is truly random which cluster might provision the bucket. This control is not provided to the
+client yet, but can be done based on the `region` parameter as supported by `AWS S3` specification.
+
+
+
+