Connection should be correctly closed even if the pool is closed

[monnand]

If the pool is closed, then closing a connection (which was get from the closed pool) will lead to a panic.

[monnand]

The following code will trigger this bug ( fakeConnManager is defined in pool_test.go ):

func TestCloseConnAfterClosingPool(t *testing.T) {
    max := 10
    manager := &fakeConnManager{nil, nil}
    pool := NewPool(max, max, manager)
    c, err := pool.Get()
    if err != nil {
            t.Errorf("Error: %v", err)
    }   

    if c == nil {
            t.Errorf("nil conn")
    }   
    // Close the pool first.
    pool.Close()

    // Then close the connection
    c.Close()
}

[monnand]

fix uniqush/uniqush-push#37