Flush the session cache when SSLContextConfig changes

TLS resumption carries over decisions from the previous connection,
including authentication steps and cipher suite decisions. This means
the TLS session cache implicitly remembers things about the previous TLS
configuration.

Flush it when SSLContextConfig changes, so the change is picked up more
reliably. This mostly only matters in edge cases. The immediate
motivation is the RSA key usage enforcement. If transitioning from
enforcement off to enforcement on, it's preferable for us to pick it up
on the next request for predictability. (Or at the least so unit tests
are easier!)

Bug: 795089
Change-Id: I1f266668930e6210d72742d15268d84df68e33cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4557005


Reviewed-by: Matt Mueller <mattm@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148024}