Keep oldest builds by default

[sdomino]

Turn off garbage collection (GC- removing oldest builds) by default.

Allow for GC to be enabled/configured via a config file providing an interval between runs, and amount to GC.

[sdomino]

there are configuration options available for this that are parsed by hoarder, however for the time being nothing is actually done with them.

All it needs is if these options are detected, is to start a goroutine that runs at the specified interval, and cleans out the specified number of records.