Run filters in lock context

Currently, unless a filter implementation explicitly puts all non-Session-method async calls in a context in which Session._broadcast is locked the filter will suffer from severe race-conditions. If Session-method calls are in such a context, a RuntimeError will be raised as locks from anyio are not re-entrant.

The methods of Session in session.py need to drop all the Broadcast locking and assume that the task they are called in has already locked it. Filters must be run in a task with the lock already acquired.

Description of work

• Remove the async context everywhere the following pattern occurs currently in session.py:

async with self._broadcast:
   ...

• Make Session usable as an async context which wraps the Broadcast

• Update runner.py to use the Session as a context