WELD-1774 Fix conversation context activation/association

- do not throw an exception when the conversation context is already
active (just log a warning that it's most likely a ThreadLocal leak)
- always associate the conversation context with the current HTTP
request
WELD-1690 Conversation lock not unlocked when ending conversations

- always try to unlock the current conversation
- don't associate a conversation which was ended
WELD-1802 An exception during context deactivation/dissociation should
not abort further processing