Clean up OnCleanup.•••- Switch to using ExtensionItem::ExtensibleType for the type instead
of TargetTypeFlags.
- Pass the extensible to OnCleanup as an Extensible pointer
instead of a void pointer.
- Call OnCleanup for memberships as well as channels and users.
- Rewrite event documentation to remove outdated references.
m_ssl_gnutls Remove DH parameter generation•••Generating DH params like this blocked the main thread, significantly slowing down startup and it made /rehash ssl freeze the server until the generation finished. External tools such as our genssl should be used to generate DH params.