DISPATCH-1130 Expose link priority

[ErnieAllen]

Adds priority to router.link management entity in the schema.
Connects link.priority to schema attribute.
Adds priority column to qdstat -l

[codecov-io]

Codecov Report

Merging #389 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   85.06%   85.02%   -0.04%     
==========================================
  Files          73       73              
  Lines       16523    16525       +2     
==========================================
- Hits        14055    14051       -4     
- Misses       2468     2474       +6

Impacted Files	Coverage Δ
src/router_core/agent_link.c	63.63% <100%> (+0.41%)	⬆️
src/router_core/core_timer.c	94.64% <0%> (-1.79%)	⬇️
src/router_core/modules/core_test_hooks.c	89.54% <0%> (-1.37%)	⬇️
src/router_core/router_core.c	92.05% <0%> (-0.34%)	⬇️
src/container.c	76.55% <0%> (-0.2%)	⬇️

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[ganeshmurthy]

@ErnieAllen This change looks good. As part of this PR, can you also please add a test to system_tests_qdstat to verify that the correct link priority is showing up? Thanks.