Robust schedule failover

[mxgrey]

This PR implements the traffic schedule failover strategy described in this comment.

This should be reviewed and merged after #86

[codecov]

Codecov Report

Merging #88 (cbd8557) into main (e60a256) will increase coverage by 0.00%.
The diff coverage is 11.11%.

@@           Coverage Diff           @@
##             main      #88   +/-   ##
=======================================
  Coverage   23.16%   23.16%           
=======================================
  Files         370      370           
  Lines       30776    30777    +1     
  Branches    14377    14377           
=======================================
+ Hits         7129     7131    +2     
+ Misses      14174    14143   -31     
- Partials     9473     9503   +30     

Flag	Coverage Δ
tests	23.04% <11.11%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
rmf_traffic/include/rmf_traffic/Route.hpp	0.00% <ø> (ø)
..._traffic/include/rmf_traffic/schedule/Database.hpp	0.00% <ø> (ø)
...mf_traffic/include/rmf_traffic/schedule/Mirror.hpp	0.00% <ø> (ø)
...mf_traffic/include/rmf_traffic/schedule/Viewer.hpp	33.33% <ø> (ø)
rmf_traffic/src/rmf_traffic/schedule/Database.cpp	49.23% <0.00%> (+0.10%)	⬆️
...fic/src/rmf_traffic/schedule/internal_Snapshot.hpp	0.00% <0.00%> (ø)
rmf_traffic/src/rmf_traffic/schedule/Mirror.cpp	41.79% <14.28%> (+0.17%)	⬆️
rmf_traffic/src/rmf_traffic/agv/Planner.cpp	52.91% <0.00%> (ø)
rmf_traffic/test/unit/agv/test_Negotiator.cpp	10.83% <0.00%> (ø)
rmf_traffic/src/rmf_traffic/DetectConflict.cpp	50.00% <0.00%> (ø)
... and 29 more