gRPC request context: activate before all the interceptors are invoked

[michalszynkiewicz]

With this change, user's global interceptors can propagate data in the request context.

[mkouba]

Looks good but I can't say that I understand all possible consequences of this change ;-)

[michalszynkiewicz]

@mkouba gRPC server first fires global interceptor, than per-service interceptor.
Before this change, the interceptor that activates the request context was a per-service interceptor. This meant that when a user created an interceptor in their code, it was invoked before the request context was activated. So no values could be put to/read from the request context in such an interceptor.

With this change, the interceptor for request context is the first global interceptor. It will always be called first :)