roll: third_party/pigweed pw_rpc: Add a test for finding a servers max echo packet size

This CL adds a new test to the pw_rpc benchmark tool.  The new test
searches for a target-side server's maximum allowed echo packet by a
doubling method until the max is found.  It enables accuracy to the
byte level, assuming the server doesn't crash.

Currently, this test is executed as part of the standard tests run
by the benchmark tool.  A future CL will expand the user interface
for selecting and running specific benchmark tests.

This CL also has some incidental comment/code cleanup noticed while
writing.

Test: added unit tests for the new benchmark test.
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/278364
Original-Revision: 6b75be9cf2bc1356bc9625637951e81695536bd1

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: bbbdf7690fead0..6b75be9cf2bc13
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8717141088387619361
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I1529049a69ec6e8e961c9e996948acac2f2fced1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/kudzu/+/284883
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 bbbdf76..6b75be9 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit bbbdf7690fead0751c35e8fdc3e936c82f253134
+Subproject commit 6b75be9cf2bc1356bc9625637951e81695536bd1