Explicit request start

[pcantrell]

#98 tried to avoid making apps ever have to explicitly start requests by using a too-clever hack involving isUniquelyReferenced. This was a bad idea: it was confusing, and precluded several kinds of request decoration one might want to do.

This PR adds a start() method to the Request protocol. Under most circumstances, apps will not have to call it. Siesta will still automatically start requests in two situations:

• Resource.request(…) (and thus load() and loadIfNeeded() as well) will start whatever request comes out of the configured decorators.
• RequestChainAction.PassTo starts the returned request.

Manual start is useful in two cases:

• A request decorator may replace original request A with replacement request B, but still hold on to A. In this case, B is automatically started, but A is not. This allows you to delay the first attempt of a request.
• Request.repeated() no longer returns an already-started request. This allows for delayed retries.