sidecar: Use Prometheus v2.2+ /api/v1/status/flags to ensure sane configuration

[bwplotka]

This will help to immediately find:

• if the sidecar tsdb.path is correct
• if local compaction is enabled or not.

[domgreen]

From Slack, compaction will begin if min duration is smaller than max.

brancz
@domgreen we can ask the prometheus server for all the flags it's running with and make the sidecar exit if 

`--storage.tsdb.min-block-duration != --storage.tsdb.max-block-duration`


brancz
if they min duration is smaller than max duration prometheus will attempt compaction