There was a problem with concurrent writes not being visible
until after restart, due to how cached prepared statements work
with SQLite (see comment in `database.cc`).
- Add support for SIGINT and SIGTERM signals for graceful termination.
- Properly check premium account status with `EVICT_FREE_ACCOUNTS`
query.
- Properly check if character is already online before failing game
login with "too many characters" error.
- Overall cleanup.
This is a bare bones query manager that will respond just enough
queries to get the server running. The overall structure of the
tables should be almost set but we'd need to handle all queries to
get the server running properly.
It uses the SQLite3 3.50.2 amalgamation for a database backend so
we can completely avoid having to spin up yet another service for a
separate database system, effectively turning the query manager into
the database itself. This also means that other services such as the
login server and website must go through it to access the data but
it shouldn't be too difficult to replicate the querying mechanism
from the server.
This design choice was made with a single game server in mind. The
networking protocol is NOT encrypted at all and won't accept remote
connections. For a fully multi-world distributed infrastructure, a
distributed database system like PostgreSQL and MySQL should be
considered.
