API report improvement / enhancement

[xeliuqa]

At the moment API for post service is very limited.
Here's a few things we think its important, specially when using monitors to report.

POS size
To get more accurate and faster results about PoST

k2pow progress status
A simple started / ended would suffice for information

PoST read status
A percentage would be best, even if in reverse 100% --> 0% we can show user how much is left

Thank you all for your hard work

[chrcao]

Regarding the service cli, I would like to add a suggestion:
Can the api methods StartSmeshing and StopSmeshing of SmesherService support the 1:n mode?
that's good to support enable/disable one of POST in full node.