Add close() method to connectionPool so connections are closed when origins are reloaded

[dvlato]

We were missing a ConnectionPool.close() method that will close open connections when an origins file is reloaded.

We are just invoking Connection::close() without modifying the metrics as, in most cases (except when the origin is removed), they would be overridden by the new SimpleConnectionPool for that origin server. We can discuss this to see what's the best way to implement it.

We can also discuss what's the best meaningful test we can implement. I guess we could add a global counter for the tota number of connections opened / closed (not per origin but global!). We might also have to locate all the code paths that should trigger a pool closure and add assertions for those in unit tests.

Of course, this PR does not prevent us from leaving connections pool that hadn't been returned to the pool (they are not in "availableConnections") at the moment the pool was closed.

[mikkokar]

Hi @dvlato this PR doesn't have any effect unless the pool consumer calls the close() method.

We need to check the following pool consumers:

1. BackendServicesProxy & OriginsInventory - This appears to close the pool correctly.
2. HostProxy - This is also okay
3. ProxyToBackend class

Going forward, the HostProxy is the only class that will survive our refactored Styx Core. The other two/three will disappear. Hopefully soon.

[mikkokar]

Of course, this PR does not prevent us from leaving connections pool that hadn't been returned to the pool (they are not in "availableConnections") at the moment the pool was closed.

I suggest: Add a closed flag in the connection pool. Set it true when the pool is closed. Then in the returnConnection check the flag and close the returned connection when the pool is closed:

    public boolean returnConnection(Connection connection) {
        borrowedCount.decrementAndGet();

        // This is the new bit:
        if (closed) {
           connection.close();
        } 

        if (connection.isConnected()) {
            queueNewConnection(connection);
        }
        return false;
    }

Of course we may have to take extra care to ensure metrics are correctly maintained.

[mikkokar]

Many thanks for this @dvlato. Very helpful :-D

Left a quick comment regarding borrowed connections. Please have a look.

Approved this regardless.

[mikkokar]

Travis fails due to check style error:

[�[1;31mERROR�[m] src/main/java/com/hotels/styx/client/connectionpool/SimpleConnectionPool.java:[99,29] (whitespace) ParenPad: '(' is followed by whitespace.