Better database connection management

[richchurcher]

With a deeper understanding of how Mongo manages its connection pool, it should be possible to be more efficient in establishing connections (and perhaps not open so many?)

It's probably worth considering whether or not the native Mongo code ./dal/store.js actually reduces complexity for workshop attendees, or whether in fact for the subject matter (APIs) it might be less distracting and error-prone to just use Mongoose (or another established tool).