Fix randomized min-timestamp interval to be stable

Previously the randomization was applied every block, which meant that the
"Waiting Xs before next tx" log message was inaccurate. Now the randomization
is applied once, after a timestamp is confirmed, by picking a time in the
future to start timestamping again.

otsserver/stamper.py

@@ -365,13 +365,14 @@ def __do_bitcoin(self):
                 # Erase all unconfirmed txs, as they all conflict with each other
                 self.unconfirmed_txs.clear()
 
-                # And finally, we can reset the last time a timestamp
-                # transaction was mined to right now.
-                self.last_timestamp_tx = time.time()
+                # Finally, schedule a new timestamp transaction.
+                #
+                # To help desync calendars, this time is randomized.
+                self.next_timestamp_tx = time.time() + (self.min_tx_interval * random.uniform(1, 2))
 
                 break
 
-        time_to_next_tx = int(self.last_timestamp_tx + self.min_tx_interval * random.uniform(1, 2) - time.time())
+        time_to_next_tx = self.next_timestamp_tx - time.time()
         if time_to_next_tx > 0:
             # Minimum interval between transactions hasn't been reached, so do nothing
             logging.debug("Waiting %ds before next tx" % time_to_next_tx)
@@ -559,7 +560,7 @@ def __init__(self, calendar, exit_event, relay_feerate, min_confirmations, min_t
         self.pending_commitments = OrderedSet()
         self.txs_waiting_for_confirmation = {}
 
-        self.last_timestamp_tx = 0
+        self.next_timestamp_tx = time.time()
         self.journal_cursor = None
 
         self.thread = threading.Thread(target=self.__loop)