Use flask and gunicorn instead of aiohttp.
This commit moves mdapi from the aiohttp framework to flask. This
is done since there is little interest in using aiohttp because all
the call to sqlite are io blocking. There is no asyncio driver for sqlalchemy
and sqlite.
Also performance with Flask and 4 gunicorn workers are better than the
performances achieved with aiohttp.
Signed-off-by: Clement Verna <cverna@tutanota.com>