Support concurrency with connection pool

[jacobg]

Currently, this plugin only supports one connection per database, whereas sqlite does support multiple connections. Check out this article for info on that:
http://charlesleifer.com/blog/sqlite-small-fast-reliable-choose-any-three-/

And because there is only one connection, contention can occur between simultaneous requests that can causes errors and even possible corruption.

The plugin should expose sqlite's support for concurrency by allowing for multiple connections, ideally even a way to manage a connection pool.

And in the meantime, perhaps the plugin should add a request queue to enforce that requests are processed one at a time, and prevent interference.

Some related issues are:

• improve API (improve API - redesign - IN PROGRESS #862)
• Multiple connections per db file (Multiple connections per db file #97)

@brodybits