Return from waiting actions when a pool is closed

Detect when a pool is closed while waiting for a connection to become
available and return an error. Otherwise the action gets stuck waiting
for a connection that won't appear and will only return when its
context's deadline is hit.

pool.go

@@ -379,6 +379,8 @@ func (p *pool) getConn(ctx context.Context) (*poolConn, error) {
 
 		select {
 		case <-p.notifyCh:
+		case <-p.proc.ClosedCh():
+			return nil, proc.ErrClosed
 		case <-ctx.Done():
 			return nil, ctx.Err()
 		}
@@ -426,6 +428,8 @@ func (p *pool) useSharedConn(ctx context.Context, a Action) error {
 
 		select {
 		case <-p.notifyCh:
+		case <-p.proc.ClosedCh():
+			return proc.ErrClosed
 		case <-ctx.Done():
 			return ctx.Err()
 		}