Add explicit initialization and running stages to web.Application

[asvetlov]

Now we have read-only tuple property named middlewares for web.Application.

It's inconvenient a bit, see example from aiohttp_debugtoolbar:

import aiohttp_debugtoolbar
app = web.Application(loop=loop,
                       middlewares=[aiohttp_debugtoolbar.middleware])
aiohttp_debugtoolbar.setup(app)

I guess to allow modify app.middlewares on setup stage (make it readwrite list property) but after first app.make_handler() call it should be converted to readonly tuple property.

Perhaps we need app.freeze() and even app.unfreeze() methods as well as app.frozen property.

router API also may be extended to allow for adding new routes only in non-frozen stage.

[fafhrd91]

-1, it is way too complicated

[fafhrd91]

just make it list

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