roll: third_party/pigweed pw_async2: Refactor channel management

This splits out channel storage from channel handles and updates channel
creation to return the user a handle. As long as a handle exists, users
can create more senders and recievers, so the channel is kept open.

Defines multiple channel handle types and factory functions which limit
the number of senders and receivers that can be created.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/339755
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: ecf027d02d7347681cbc855b4d579d6967abacd2

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: bd5e1ca1f5e491..ecf027d02d7347
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8699445108382893681
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ida741dd8146261c33fb0c8ccb29bffe6570d3f01
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/340252
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index bd5e1ca..ecf027d 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit bd5e1ca1f5e491b9ea113946f0072118f0bd87c6
+Subproject commit ecf027d02d7347681cbc855b4d579d6967abacd2