Fix for possible ftp connection pool starvation.

[s4ibot]

There is a possible connection pool starvation in methods FTPClientPool.get() and FTPClientPool.getOrCreate(). The isConnected() check should be fail-safe, otherwise there is a possibility that the Client will never return to the pool.

In my case it's also important to check the connection with a keepAlive() call. Because my ftp server can drop idle connections.

[robtimus]

Is this not needed anymore? Because I was planning on working on it this weekend.

[s4ibot]

The pool starvation is still there. But my commits don't fix the problem.

[robtimus]

I've just committed a change that should prevent pool starvation. Could you test if this works?

I'm also thinking about using Apache Commons Pool instead, but that would add more configuration features that need consideration.

[s4ibot]

Thanks!
I will take a look on the changes.
I thought about Apache Commons Pool too, but imo it would be an overkill.