swarm/src/behaviour: Remove Send bound from NetworkBehaviour

[dignifiedquire]

Needs confirmation this is okay design wise, but asfaict this is not actuallly required for the core of libp2p to work.

cc @mxinden

[mxinden]

I am considering this a non-breaking change, thus the patch version bump only. Please comment in case you think otherwise.

[thomaseizinger]

Technically it is breaking.

People could rely on the implicit Send bound if they write code that it is abstract over NetworkBehaviour and sent across thread boundaries.

I think that can fairly easily happen if someone builds a generic piece of code for spawning a Swarm into a task pool as we can see from the tests.

I think publishing it as breaking change would be better. I don't know if any library code exists that does depend on this but if it does, having things break in a dependency is a pretty shitty experience.

[dignifiedquire]

agree with @thomaseizinger , it will likely require some folks to change their trait bounds if they are generic over it.

[mxinden]

Thanks @dignifiedquire for the find.