New `EventTracker` methods

[danfuzz]

[Moving an in-source TODO to an issue.]

Consider adding these methods to async/EventTracker:

• nextNow() -- Synchronous version of next().
• peekNow() -- Synchronous version of peek().
• withPushedHead() -- Produce a new instance as described.
• withReplacedHead() -- Produce a new instance as described (uses event.withPayload()).