Implemented several workarounds to what appearto be a library bug in
which not all users will be downloaded onto the cache. User cache
checking and downloading is delegated to a new Common method and a
new BackgroundService, respectively.
This is hopefully temporary.
The BirthdayBot class has been split up into ShardInstance and
ShardManager. Several other things have been reorganized so that shards
may act independently.
The overall goal of these changes made is to limit failures to sections
that can easily be discarded and replaced.
