Add check in wait_lsn for nil value

Found issue in testing:

  [006] --- replication/election_qsync.result	Fri Oct 16 00:12:02 2020
  [006] +++ replication/election_qsync.reject	Sat Oct 17 19:08:11 2020
  [006] @@ -89,6 +89,8 @@
  [006]  -- Wait replication to the other instance.
  [006]  test_run:wait_lsn('default', 'replica')
  [006]   | ---
  [006] + | - error: '...sitories/tarantool/test/var/006_replication/test_run.lua:68: attempt
  [006] + |     to compare nil with number'
  [006]   | ...

Found that wait_lsn() routine from test_run.lua script failed on
comparing returned result from get_lsn() routine with real integer.
It happened because get_lsn() returned 'nil' on the freshly
bootstrapped instance. To avoid of it in wait_lsn() routine added
check that get_lsn() routine returned not 'nil' value.
Complete explanation of the issue and reproducer provided in [1].

Closes #226

Co-authored-by: Alexander Turenko <[email protected]>

[1]: #269 (comment)

Author: avtikhon

test_run.lua

@@ -65,9 +65,10 @@ local function wait_lsn(self, waiter, master)
     local sid = self:get_server_id(master)
     local lsn = self:get_lsn(master, sid)
 
-    while self:get_lsn(waiter, sid) < lsn do
+    repeat
         fiber.sleep(0.001)
-    end
+        local waiter_lsn = self:get_lsn(waiter, sid)
+    until waiter_lsn ~= nil and waiter_lsn >= lsn
 end
 
 local function get_vclock(self, node)