Option allows to limit a frame by size before decoding its payload.

LH: I have added a test for when the limit is reached on a nofin
fragmented frame (the last commit addressed that case but it had
no test). I have fixed formatting and other, and changed the
default value to infinity since it might otherwise be incompatible
with existing code. I also added documentation and a bunch of other
minor changes.

We now flush messages that are specific to cowboy_http only.

Stream handlers should also flush their own specific messages
if necessary, although timeouts will be flushed regardless
of where they originate from.

Also renames the http_SUITE to old_http_SUITE to distinguish
new tests from old tests. Most old tests need to be removed
or converted eventually as they're legacy tests from Cowboy 1.0.

Also rename a bunch of functions to make the code easier to read.

Currently cowboy assumes that idle_timeout or request_timeout is
a number and always starts timers. Similar situation takes place
in case of preface_timeout for http2. This commit adds case for
handling infinity as a timeout, allowing to not start mentioned
timers.