refactor(facade): separate redis data/control paths (phase 2m final)

Separates Redis pipeline processing from the SendAsync control path to
improve architecture and allow distinct monitoring.

* Fixes pipeline_buffer_limit enforcement by introducing
QueueBackpressure::pipeline_bytes to track global per-thread memory
usage, resolving a bug where limits were applied per-connection.
* Adds a safety barrier in ClearPipelinedMessages to wait for in-flight
async commands during teardown, preventing use-after-free races.
* Fixes potential underflow assertions in ReleaseParsedCommand.
* Optimizes fiber notification logic based on protocol specific needs.

Signed-off-by: Gil Levkovich <[email protected]>

Author: glevkovich