client/API/docker_wrapper: get CPU and mem usage of Docker apps

[davidpanderson]

Mem usage (WSS):
The easiest way to get the WSS of a Docker contaier is to ask Docker
using the "docker stats" command.
So I have docker_wrapper do this periodically (10 sec... it's a bit slow).

But how to get this back to the client?
Currently there's no provision for an app to reports its own WSS.
So I added one, by adding an optional field to the app status messages
sent from app to client in shared mem.
If this is present, the client uses it instead of procinfo.

CPU time: "docker stats" reports CPU fraction
(averaged over what period?)
We multiply that by the stats poll.
Not exactly the same as CPU time, but close enough.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 10.75%. Comparing base (96370db) to head (984d0ab).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
api/boinc_api.cpp	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5870      +/-   ##
============================================
- Coverage     10.75%   10.75%   -0.01%     
  Complexity     1068     1068              
============================================
  Files           280      280              
  Lines         36552    36555       +3     
  Branches       8471     8472       +1     
============================================
  Hits           3930     3930              
- Misses        32233    32236       +3     
  Partials        389      389              

Files with missing lines	Coverage Δ
api/boinc_api.h	0.00% <ø> (ø)
api/boinc_api.cpp	0.00% <0.00%> (ø)