Slow shutdown allows one to end up with 2 memcached instances

From MB-1422...

This bug is with the issue of having 2 memcached processes running, from Perry...

There are few behavioral issues to address here, perhaps better split into
different bugs, I'm not sure.

Using brutis (./brutis -x 10.1.3.97 -r 1:0 -s 1024 -k 1000000 -v) I generated
about 500k keys, and about 1GB of memory (as reported by top) into memcached.

I then ran the 'flush_all' command, waited for it to return 'OK' and
immediatley stopped the service (/etc/init.d/northscale-server stop).

The memcached process remains running and spikes the CPU for some time (I'm
assuming it's writing all the data out to disk). This is the first "undesired"
behavior from a user's perspective. When we report that the service is
stopped, I as the user would expect that the processes are no longer running.
I would suggest at this point that we pause before returning the 'OK' for
stopping the service.

The second step involves starting the service again while the original
memcached process is still running. You get a "Failed to start...". I tried
it again, you get a "Warning, it is already started".

Looking at top, I now see two memcached process, the original one and a new
one.

The new one begins filling up with data (I'm assuming whatever has been written
to disk but not yet flushed) and telnetting to 11211 allows a connection but
hangs when trying to execute any command (including stats).

This is the second "undesired" behavior. Restarting the process should not
start up a new memcached instance, and it certainly shouldn't fill itself with
data that was supposed to be flushed. Of utmost importance is that if the
instance is up, it is responsive to clients. This is not the case. For this,
I would suggest preventing the startup of an instance if there is already one
running.