support header namespace isolation

[pingliu]

[] support header namespace isolation

[wu-sheng]

This had been removed from the protocol months ago. We don't support this anymore.

Ref apache/skywalking-java#123

Change log

• [Breaking Change] Remove the namespace from cross process propagation key.

The points we did are

1. This is never defined in the protocol/APIs.
2. Breaking a distributed tracing seems unreasonable because they are really accessing each other.
3. In four years of history, there is no other agent tried to support this, and we think <1> and few use cases are the reasons.

[pingliu]

ok