fix(context): clear call context when done

[ziadsawalha]

Some threads can be reused by a bottle server and
old call data can persist in the thread-local
storage. This ensures the data is cleared after
each request.

[stavxyz]

The threadlocal call context will now be unavailable to any middlewares "above" the ContextMiddleware, right? Whereas previously, the context was available for the life of the thread. I don't think we've ever had code in that logical place that would depend on the threadlocal call context, but I wanted to make sure I understood correctly and document the caveat if there is one.

[ziadsawalha]

The threadlocal data is available to anything in the thread. However, the .default() context will be cleared after the ContextMiddleware finishes handling its response. I make the assumption that any workers or asynchronous code running is in another thread/process/machine and would have had the context copied over by then.

Given the ContextMiddleware is what returns the X-Transaction-Id header, it should be very close to the top of the stack (if not at the top). Since any error or failure should go back with the X-Transaction-Id header in it as an ID we can use for further troubleshooting.

[ziadsawalha]

Thanks @samstav