v0.5.2 - hotfix for database issues

99 bugs in the code, 99 bugs in the code, take one down patch it around, 127 bugs in the code...

No fancy release title as this is really just a hotfix.
https://github.com/modrzew/pokeminer/releases/new#

• no more pool_size for create_engine (reverts 3a775e5) - it didn't work out of the box for SQLite. Moreover, there's a different solution that will be implemented soon.
• no more grouping by id in get_forts (reverts d0cb311) - this made query return all fort sightings, instead of only the recent ones.