feat: support pebble datastore

[gammazero]

Added cli flags for all peble configuration options. Setting any to the zero-value for the type will cause Pebble's devauld value to be used.

[lidel]

thanks! looks sensible enough to do A/B test in staging before continuing. I mainly want to see if Pebble's internal compaction is enough for Rainbow's use case, or do we need to explicitly call it in gc.go

[gammazero]

I mainly want to see if Pebble's internal compaction is enough for Rainbow's use case, or do we need to explicitly call it in gc.go

There is no need to explicitly do compaction in a GC cycle with pebble. If there is a very high constant influx of data, and the compaction is not catching up, then tuning parameters can be adjusted as needed.