Move message result management to PlannerClient

[csegarragonz]

The message results are managed by the planner, so there's no need for the setting/getting of those messages to be part of the Scheduler class.

Instead, in this PR I move them to the PlannerClient. Getting/setting a message involves, most of the times, to just sending a request to the PlannerServer. To implement client-side blocking we have a small message result cache, which we also integrate in the PlannerClient.

[codecov]

Codecov Report

Merging #340 (06cdbe6) into main (b81cdaa) will decrease coverage by 0.24%.
The diff coverage is 89.70%.

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
- Coverage   82.04%   81.81%   -0.24%     
==========================================
  Files          98       98              
  Lines        7972     7966       -6     
==========================================
- Hits         6541     6517      -24     
- Misses       1431     1449      +18     

Files Changed	Coverage Δ
include/faabric/scheduler/Scheduler.h	42.85% <ø> (ø)
src/planner/PlannerClient.cpp	83.96% <87.50%> (+0.62%)	⬆️
src/planner/PlannerServer.cpp	89.39% <100.00%> (+4.54%)	⬆️
src/runner/FaabricMain.cpp	93.47% <100.00%> (-0.14%)	⬇️
src/scheduler/FunctionCallServer.cpp	79.68% <100.00%> (ø)
src/scheduler/Scheduler.cpp	91.10% <100.00%> (-0.58%)	⬇️
src/util/ExecGraph.cpp	62.69% <100.00%> (ø)

... and 2 files with indirect coverage changes