Starting to allow for >1 implementation of pollset within a binary.
Do so without requiring an extra allocation for completion queues (which
we could not tolerate).
Allow plugins to extend the set of filters used by gRPC core:
- plugins at construction time can register against the 'channel_init' system to be allowed to mutate a new channel_stack_builder type
- channel_stack_builder provides a central and rather dynamic place to construct the list of filters required by a channel stack
- ultimately we construct the channel stack in the fashion we always have
This is also a prerequisite step to allowing filters to be implemented from wrapped languages.
Specifically:
Receiving trailing and initial metadata had to be published in
lock-step.
=> If we wanted trailing metadata, we might not get initial metadata processed
until messages arrived.
=> Compression code had no idea what codec to use.
To fix it, publish initial metadata as soon as it's ready (this is a
transport API change).
Requires changes to grpc_call to ensure ordering in processing initial
metadata and messages (one may be delayed).
Exposed at least some bugs in C++ where we never read initial metadata.
I expect at least one more similar bug.