Remove spec-related fields from stat.
We can simplify the stats a bit further by handling Int and Float better.
But this was big enough change already.
Verified v1 and v2 spec/stats/appmetrics APIs.
Docker does not provide the rootfs path through docker inspect or statefile
and the path is dependent on the storage driver being used.
Instead of enumerating the storage drivers, we pick a pid from the container
and get the config from /proc/pid/root. Although a bit expensive, this method
works for non-docker containers too.
Since polling is tied to housekeeping, minimum supported polling
frequency is 1s.
Users can specify polling frequency higher than 1s. The polling loop
will be called at the minimum frequency specified in config as long as
its higher than the minimum supported frequency.
