Backup API from driver

[ncruces]

The backup API can be used with the database/sql driver, but it is not particularly ergonomic to do so. That's the reason behind #141.

Provide a more ergonomic alternative.

In particular, one that implements the more complicated, and harder to get right, use case: if SetMaxOpenConns is 1 for this database, do the backup incrementally, releasing the lock every few pages.