- Run the cleanup code on exceptions (e.g. ^C). There were several effects of that not happening previously; most notably, the log file was not written to the meta WARC.
- Cancel remaining tasks, which avoids a pile of asyncio warnings and errors on crashes.
- Close the DB before the WARC, or rather, close the WARC last. This is mostly a semantic change to further ensure that the log written to the meta WARC is as complete as possible.

The DB creation operates with a DB lock, so that code can't run while another process is filling the DB; it would block on obtaining the lock a few lines prior instead.

It is rarely necessary to access the history, and the tuple return value clutters the spec file code.

As a consequence, it's no longer possible to return None if an error occurred without losing the history.
To replace that, this also introduces a DummyClientResponse, which is kind of ClientResponse-like, has the same qhistory attribute, and evaluates to False when cast to bool (such that the intuitive `if response` works as expected).

Since Python 3.8, the latter does not inherit from the former anymore.

The asyncio documentation does not specify whether awaiting a Future multiple times is supported or not: https://bugs.python.org/issue41275

If an exception was raised and caught, the object is still present in the history.

This also renames add_item to add_subitem for clarity.

chardet can be very slow (https://github.com/chardet/chardet/issues/29 https://github.com/psf/requests/issues/2359) and the decoding may be unnecessary if it's binary content.