Rename AttributeKey persistent channel to allow coexisting with 0.7

[smaldini]

If not renamed, having both 0.7 and 0.8 in use in your application will result in:

    at io.netty.util.ConstantPool.createOrThrow(ConstantPool.java:113) ~[netty-common-4.1.29.Final.jar:4.1.29.Final]
    at io.netty.util.ConstantPool.newInstance(ConstantPool.java:95) ~[netty-common-4.1.29.Final.jar:4.1.29.Final]
    at io.netty.util.AttributeKey.newInstance(AttributeKey.java:55) ~[netty-common-4.1.29.Final.jar:4.1.29.Final]
    at reactor.netty.ReactorNetty.<clinit>(ReactorNetty.java:614) ~[reactor-netty-0.8.0.RELEASE.jar:0.8.0.RELEASE]```