buildslaves db and data

Add DB support for tracking where buildslaves are configured and where they are connected, and a basic data API layer atop that.

This models buildslaves as connected to zero or more masters, and configured on zero or more (builder, master) pairs.

This also goes to some pains to:
 * limit buildslave names to 50-character identifiers
 * use the word "bulidslave" rather than "slave" wherever possible