Extending the context instead of fixing the API breaks type safety.
For tracking the number of points / values written, it is much clearer
to pass an explicit tracker.
Test the skipCacheOk flag to tsdb.Shard.CreateSnapshot() and
tsdb.Engine.CreateSnapshot()
A value of true allows the backup to proceed even if a cache
snapshot cannot be taken.
https://github.com/influxdata/plutonium/issues/3227