Rename internal hooks

[mafintosh]

After hitting a few "user errors", I think we should rename the following internal hooks for clarity (semvar major, but public api the same)

• _destroy (cb) --> _close(cb)
• _predestroy() -> _destroy(err)

_close to better reflect that its the opposite of _open and _destroy to indicate that this is the "intercept the statemachine and destroy as soon as you can" hook.