The vast majority of the virtual methods which were used on ProtocolState were
actually methods which only had meaningful implementations on the toplevel
protocol. This patch adds a new field to IProtocol holding a direct pointer to
the protocol's `IToplevelProtocol`, and the methods formerly implemented with
ProtocolState now directly call the corresponding method on IToplevelProtocol.
IToplevelProtocol then shadows these methods with the toplevel protocol
implementation, meaning that the right code is run in the right places.
In addition, some state was maintained for protocols inside of the separate
ProtocolState allocation, and this patch moves that state back into the actor
itself.
Differential Revision: https://phabricator.services.mozilla.com/D32044
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