Backport #1141's flaky-test fixes to the stable branches

[petergoldstein]

#1141 (115ded5) fixed three flaky tests on main:

1. KeyManager: in "when the key with namespace is shorter than 250 characters", the generated key length didn't account for the : separator, so about 1 run in 250 produced a 251-character key, which gets hashed and fails the assertion.
2. The Rack session freshness test raced its 1-second TTL.
3. MemcachedManager: failed_start could leave a process holding a port, and stop didn't confirm the port was released.

5-1-stable has the fix. 3-2-stable, 4-3-stable and 5-0-stable don't, and the KeyManager flake failed release PRs this week:

• Prepare 3.2.11 release #1176 (3.2.11), Ruby 3.0 / Memcached 1.5.22
• Prepare 5.0.8 release #1174 (5.0.8), JRuby 10 / Memcached 1.6.41

Backporting 115ded5 to those three branches should remove all three flakes there.

[petergoldstein]

Backports are up: #1181 (5-0-stable), #1182 (4-3-stable) and #1183 (3-2-stable). The cherry-pick applied cleanly on 5.0.x and 4.3.x. On 3.2.x the Rack session test had diverged (a 3-second TTL and the older assertion style), so I ported the split by hand.

[petergoldstein]

Done. #1141's fixes are now on all the stable branches: #1181 (5-0-stable), #1182 (4-3-stable) and #1183 (3-2-stable). Each has a changelog entry under Unreleased → Development.