Mojo: Shared thread-local node for DirectReceivers

Reworks DirectReceiver to use a shared thread-local ipcz node which is
lazily brought up and torn down as needed, rather than a dedicated node
per receiver.

This allows multiple receivers on the same thread to reuse the same node
and therefore the same underlying I/O transports to other nodes.

Bug: 1441977
Change-Id: I78a87f65780584341737f5ee47e6a394eaf9e302
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4549245


Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/main@{#1147382}