Support access format for database notification targets (#3953)

* Add configuration parameter "format" for db targets and perform
  configuration migration.
* Add PostgreSQL `access` format: This causes Minio to append all events
  to the configured table. Prefix, suffix and event filters continue
  to be supported for this mode too.
* Update documentation for PostgreSQL notification target.
* Add MySQL `access` format: It is very similar to the same format for
  PostgreSQL.
* Update MySQL notification documentation.

docs/config/config.sample.json

@@ -53,6 +53,7 @@
         "elasticsearch": {
             "1": {
                 "enable": true,
+                "format": "namespace",
                 "url": "http://127.0.0.1:9200",
                 "index": "bucketevents"
             }
@@ -60,6 +61,7 @@
         "redis": {
             "1": {
                 "enable": true,
+                "format": "namespace",
                 "address": "127.0.0.1:6379",
                 "password": "yoursecret",
                 "key": "bucketevents"
@@ -68,6 +70,7 @@
         "postgresql": {
             "1": {
                 "enable": true,
+                "format": "namespace",
                 "connectionString": "",
                 "table": "bucketevents",
                 "host": "127.0.0.1",
@@ -93,6 +96,7 @@
         "mysql": {
             "1": {
                 "enable": true,
+                "format": "namespace",
                 "dsnString": "",
                 "table": "minio_images",
                 "host": "172.17.0.1",