Fix wait_for_downstream_to

Currently `wait_for_downstream_to` doesn't wait for replica to be up
to date with master. It only checks, that downstream "knows" correct
vclock of the replica by taking vclock of remote server and waiting
for info.downstream.vclock to be the same. The function's comment says
the opposite, so this is not expected behavior.

Let's take local vclock instead. This way the function will wait for
remote server to apply all master's data. Moreover, let's update
function's comment to be less vague.

Closes tarantool/tarantool-qa#311

Author: Serpentian

luatest/server.lua

@@ -858,12 +858,13 @@ function Server:wait_for_vclock(vclock)
     end
 end
 
---- Wait until all own data is replicated and confirmed by the given server.
+--- Wait for the given server to reach at least the same vclock as the local
+-- server. Not including the local component, of course.
 --
 -- @tab server Server's object.
 function Server:wait_for_downstream_to(server)
     local id = server:get_instance_id()
-    local vclock = server:get_vclock()
+    local vclock = self:get_vclock()
     vclock[0] = nil  -- first component is for local changes
     while true do
         if vclock_ge(self:get_downstream_vclock(id), vclock) then