Session cache should keep track of TLS version maximum

In the BoringSSL [documents on session caching](https://commondatastorage.googleapis.com/chromium-boringssl-docs/ssl.h.html#Session-caching), it states:

> Note that offering or accepting a session short-circuits most parameter negotiation. Resuming sessions across different configurations may result in surprising behavior. So, for instance, a client implementing a version fallback should shard its session cache by maximum protocol version.

Conscrypt should be changed to track maximum protocol version in its SSL session cache to avoid these gotchas.