master: fix possible failure when reading data in a backup cluster with replicas

Given that:
- read locks are only taken by transactions (not replication)
- in backup mode, storage nodes stay in UP_TO_DATE state, even if partitions
  are synchronized up to different tids

there was a race condition with the master node replying to LastTransaction
with a TID that may not be replicated yet by all replicas, potentially causing
such replicas to reply OidDoesNotExist or OidNotFound if a client asks it data
too early.

IOW, even if the cluster does contain the data up to `getBackupTid(max)`,
it is only readable by NEO clients up to `getBackupTid(min)` as long as the
cluster is in BACKINGUP state.

neo/master/handlers/client.py

@@ -147,5 +147,5 @@ class ClientReadOnlyServiceHandler(ClientServiceHandler):
     # like in MasterHandler but returns backup_tid instead of last_tid
     def askLastTransaction(self, conn):
         assert self.app.backup_tid is not None   # we are in BACKUPING mode
-        backup_tid = self.app.pt.getBackupTid()
+        backup_tid = self.app.pt.getBackupTid(min)
         conn.answer(Packets.AnswerLastTransaction(backup_tid))