Add escape hatches for DatabaseResultRow type coercing

JDBC seems to have involved a random number generator in deciding
whether or not to return a Long for integer-based fields. Be safer in
this regards.
Load account information from the database

When a player connects to Mineground, their account information should
be automatically read from the database.

This patch does a few more random things:
* Promise.reject() now accepts a string, for convenience.
* AccountData gained a constructor for setting default values.