Early shutdown causes panic

To replicate, run the client and ctrl+c to quit during "Load blockchain..."

gnet/pool will panic with a nil `*Connection`

This is because of `strand("NewConnection",`, `pool.NewConnection` may return `nil, nil` unexpectedly (it should always be `non-nil, nil` or `nil, err`.

`strand` returns `nil` when quit. It should return an `ErrQuit`. However, when I changed it to return `ErrQuit`, the program hung on shutdown instead.

`strand` should return `ErrQuit` so that `strand` callers can expect that they received an error if their method was not called before returning. The program hang that occurs when `ErrQuit` is returned must be solved as part of this.