ThreadLocal Hubs should be cleaned up

[adinauer]

Problem Statement

At the moment Hub instances are stored in ThreadLocal variables. They are never cleaned up and thus cause servers like Tomcat to issue warnings as can be seen here #2074, here #420 and here #487. While there are mechanisms to clean up these leaks in Tomcat we should not rely on them and clean up instead.

sentry-java/sentry/src/main/java/io/sentry/Sentry.java

Line 21 in 9f1af1c

private static final @NotNull ThreadLocal<IHub> currentHub = new ThreadLocal<>();

Solution Brainstorm

We may have to move away from ThreadLocal variables at some point to support Webflux and similar. Until then we could try and unset the ThreadLocal after a request is done (ServletRequestListener.requestDestroyed)

[bruno-garcia]

Closed through #2076 (somehow GitHub didn't automatically close it)

[adinauer]

#2076 only allows Threads that had any Sentry.* method called before Sentry.init to self heal. This issue is still unresolved.

[adinauer]

We could do it similar to this: sentry/src/main/java/io/sentry/servlet/SentryServletRequestListener.java