Implement ProtocolFeatureStore

[dkropachev]

Implement class to store information about all features negotiated.
The main idea is to have a single place where we store that information, instead of spreading all over channel, and ProtocolInitHandler

It is required by #599

[Bouncheck]

LGTM.

There may be very slight performance hit, probably negligible.
Instead of getting specific features directly the getters will now get whole feature store and then the specific feature from it.

[Bouncheck]

I see that two features had method names changed to populateStartupOptions and loadFromSupportedOptions. If you'd like to enforce a convention for the future features consider making an Interface and turning the existing ones into implementation of it

[dkropachev]

I see that two features had method names changed to populateStartupOptions and loadFromSupportedOptions. If you'd like to enforce a convention for the future features consider making an Interface and turning the existing ones into implementation of it

I was thinking about the same, loadFromSupportedOptions is static, so it can't go into interface and having interface for only one method does not make sense, so I did not do it.