Increase default minimum tx interval to 6 hours

With the four public calendars operating independently, this should still
result in multiple timestamps per day. Only one is needed, so this should still
provide better time precision almost all the time than the ~1 day that you can
reasonably trust Bitcoin mining to provide without more careful analysis.

otsd

@@ -75,7 +75,7 @@ parser.add_argument("--btc-min-confirmations", metavar='N', type=int,
                     help="Confirmations required before we save a Bitcoin timestamp permanently, must be greater than 1"
                          " (default: %(default)d)")
 parser.add_argument("--btc-min-tx-interval", metavar='SECONDS', type=int,
-                    default=3600,
+                    default=60*60*6,
                     help="Minimum interval between timestamp transactions (default: %(default)d seconds)")
 parser.add_argument("--btc-max-fee", metavar='FEE', type=float,
                     default=0.001,