Add support for dynamically disabling writes and redirects

[mdehoog]

Example config:

{
  "Clusters": {
    ":12345": {
      "DisableWrites": true,
      "RedirectTo": ""
    },
    "/var/tmp/mongo_accounts.sock": {
      "DisableWrites": true,
      "RedirectTo": "/var/tmp/mongo_accounts_sharded.sock"
    }
  }
}