Improve logic of tracer::aquire_tool_id

[tzanko-matev]

Currently we always use tool_id=5. This has the following problems:

• The user is unable to set their own tool id if they wish. This makes it difficult for them to attach several tools at the same time
• tool_id = 5 means that our tool is an optimizer by the convention described in https://docs.python.org/3/library/sys.monitoring.html. A debugger is more appropriate
• If the tool_id 5 is already taken right now we just fail. It is not possible to register our tracer with another tool_id.

All of this will lead to problems once people start using our tracer in the wild. We should improve the design before then.