Clean out the LogContext childContexts array when a new subcontext is added.

Should prevent unbounded growth of the context array for contexts with lots of
short-lived children.

Author: JonathanLennox

src/main/kotlin/org/jitsi/utils/logging2/LogContext.kt

@@ -63,6 +63,7 @@ class LogContext private constructor(
         ancestorsContext + context,
         childContextData
     ).also {
+        childContexts.removeIf { r -> r.get() == null }
         childContexts.add(WeakReference(it))
     }