RFR: 8298381: Improve handling of session tickets for multiple SSLContexts

[Volker Simonis]

Currently, TLS session tickets introduced by [JDK-8211018](https://bugs.openjdk.org/browse/JDK-8211018) in JDK 13 (i.e. `SessionTicketExtension$StatelessKey`) are generated in the class `SessionTicketExtension` and they use a single, global key ID (`currentKeyID`) for all `SSLContext`s.

This is problematic if more than one `SSLContext` is used, because every context which requests a session ticket will increment the global id `currentKeyID` when it creates a ticket. This means that in turn all the other contexts won't be able to find a ticket under the new id in their `SSLContextImpl` and create a new one (again incrementing `currentKeyID`). In fact, every time a ticket is requested from a different context, this will transitively trigger a new ticket creation in all the other contexts. We've observed millions of session ticket accumulating for some workloads.

Another issue with the curent implementation is that cleanup is racy because the underlying data structure (i.e. `keyHashMap` in `SSLContextImpl`) as well as the cleanup code itself are not threadsafe.

I therefor propose to move `currentKeyID` into the `SSLContextImpl` to solve these issues.

The following test program (contributed by Steven Collison (https://raycoll.com/)) can be used to demonstrate the current behaviour. It outputs the number of `StatelessKey` instances at the end of the program. Opening 1000 connections with a single `SSLContext` results in a single `StatelessKey` instance being created:

$ java -XX:+UseSerialGC -Xmx16m -cp ~/Java/ SSLSocketServerMultipleSSLContext 9999 1 1000
605:             1             32  sun.security.ssl.SessionTicketExtension$StatelessKey (java.base at 20-internal)

The same example with the 1000 connections being opened alternatively on thwo different contexts will instead create 1000 `StatelessKey` instances:

java -XX:+UseSerialGC -Xmx16m -cp ~/Java/ SSLSocketServerMultipleSSLContext 9999 2 1000
  11:          1000          32000  sun.security.ssl.SessionTicketExtension$StatelessKey (java.base at 20-internal)

With my proposed patch, the numbers goes back to two instances again:

java -XX:+UseSerialGC -Xmx16m -cp ~/Java/ SSLSocketServerMultipleSSLContext 9999 2 1000
  11:          1000          32000  sun.security.ssl.SessionTicketExtension$StatelessKey (java.base at 20-internal)


I've attached the test program to the [JBS issue](https://bugs.openjdk.org/browse/JDK-8298381). If you think it makes sense, I can probably convert it into a JTreg test.

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Commit messages:
 - 8298381: Improve handling of session tickets for multiple SSLContexts

Changes: https://git.openjdk.org/jdk/pull/11590/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11590&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298381
  Stats: 123 lines in 2 files changed: 54 ins; 58 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/11590.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11590/head:pull/11590

PR: https://git.openjdk.org/jdk/pull/11590