discord: retry auth/command register, do latter in bulk

re-use the auth retry options from the config for initial d.bot.Open and
for registering commands. The latetr is now done with the BulkOverwrite
    method, since it seems to work now. For #427.

common/common.go

@@ -11,6 +11,7 @@ import (
 	"net/http"
 	"net/url"
 	"strings"
+	"time"
 
 	lm "github.com/hrfee/jfa-go/logmessages"
 )
@@ -155,3 +156,11 @@ func decodeResp(resp *http.Response) (string, error) {
 	}
 	return buf.String(), nil
 }
+
+// MustAuthenticateOptions is used to control the behaviour of the MustAuthenticate-like methods.
+type MustAuthenticateOptions struct {
+	RetryCount  int           // Number of Retries before failure.
+	RetryGap    time.Duration // Duration to wait between tries.
+	LogFailures bool          // Whether or not to print failures to the log.
+	Counter     int           // The current retry count.
+}