Add support to cut an active block based on total bytes

[joe-elliott]

Is your feature request related to a problem? Please describe.
Currently Tempo allows for cutting the active block based on time or total traces. However trace size can vary wildly. This should allow for cutting the active block by total current bytes.

https://github.com/grafana/tempo/blob/master/modules/ingester/config.go#L20-L21

Describe the solution you'd like
Add a config option (max_block_bytes ?) that cuts a block when the threshold is reached. I think we should remove traces_per_block in favor of this setting.