fix unstable replication test

this test which has coverage for varoius flows of diskless master was
failing randomly from time to time.

the failure was:
[err]: diskless all replicas drop during rdb pipe in tests/integration/replication.tcl
log message of '*Diskless rdb transfer, last replica dropped, killing fork child*' not found

what seemed to have happened is that the master didn't detect that all
replicas dropped by the time the replication ended, it thought that one
replica is still connected.

now the test takes a few seconds longer but it seems stable.

Author: oranagra

tests/integration/replication.tcl

@@ -513,9 +513,9 @@ start_server {tags {"repl"}} {
     set master_port [srv 0 port]
     set master_pid [srv 0 pid]
     # put enough data in the db that the rdb file will be bigger than the socket buffers
-    # and since we'll have key-load-delay of 100, 10000 keys will take at least 1 second
+    # and since we'll have key-load-delay of 100, 20000 keys will take at least 2 seconds
     # we also need the replica to process requests during transfer (which it does only once in 2mb)
-    $master debug populate 10000 test 10000
+    $master debug populate 20000 test 10000
     $master config set rdbcompression no
     # If running on Linux, we also measure utime/stime to detect possible I/O handling issues
     set os [catch {exec unamee}]