DISPATCH-1345: coalesce QDR_LINK_WORK_FLOW events

[kgiusti]

avoid adding another QDR_LINK_WORK_FLOW work item if there is already one at the tail of the link's work list.

[ganeshmurthy]

Nice change !

[codecov-io]

Codecov Report

Merging #522 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
- Coverage   86.85%   86.84%   -0.01%     
==========================================
  Files          87       87              
  Lines       19507    19518      +11     
==========================================
+ Hits        16942    16950       +8     
- Misses       2565     2568       +3

Impacted Files	Coverage Δ
src/router_core/transfer.c	94.6% <100%> (+0.17%)	⬆️
...re/modules/edge_addr_tracking/edge_addr_tracking.c	84.94% <0%> (-1.62%)	⬇️
...c/router_core/modules/test_hooks/core_test_hooks.c	92.67% <0%> (-1.28%)	⬇️
src/router_core/router_core.c	86.73% <0%> (+0.19%)	⬆️
src/iterator.c	89.23% <0%> (+0.19%)	⬆️
src/parse.c	85% <0%> (+0.23%)	⬆️
src/router_core/agent_link.c	68.04% <0%> (+0.51%)	⬆️

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