Tablets: introduce system pipes and use them for http requests

[snaury]

Http requests currently use regular pipes, which don't connect until tablets have fully initialized and activated. In practice, when tablets experience problems in their initial transactions it becomes impossible to peek into their internal tables and figure out what exactly went wrong.

I think we need to introduce "system" pipes that don't wait for activations and are somewhat invisible on the "user" side of the tablet. Then http requests would be forwarded via system pipes, and messages will arrive to the system part of the tablet. It would in turn either handle requests on its own (e.g. when user part is unavailable it could draw basic index page with some basic status, otherwise enqueue it until it's ready), and forward them to user part otherwise.

A challenge would be to support handling http requests in some very early states, when executor is not ready yet, restoring the state, etc. Many tablets currently use transactions to handle requests, but transactions don't work until tablet is active, so this should be handled carefully somehow.