Prepare notify 7.0 release

[dfaust]

The change of the event serialization (#558) requires a release of a new major version. Are there other breaking changes that we should make before rolling a new release?

My suggestion:

• Remove crossbeam-channel from the default features

[0xpr03]

#525 possibly

[0xpr03]

Also +1 for removing crossbeam channel. It's a safe bet that it will be part of the std anyway.

[kaimast]

It would be great if you could update to mio 1.0 for the next release.