_context.py: Adding apis for message recording and task logging.

Added the new Context.recorded_messages() context manager, this
causes messages to be logged in a dedicated log file before being
propagated to the log handler, and also provides an open log handle
for the capturing and logging of stdout/stderr from child processes.

This comes with the additional accessors:

   Context.get_log_handle()
   Context.get_log_filename()