DISPATCH-1475: prevent I/O thread from accessing link during deletion

[kgiusti]

Fix includes consistently adding link to proper priority slot in
connections links_with_work array.

[ChugR]

This commit works well. I've blasted my setup for a couple of hours, 250,000 reconnects, all with no issue. Ship it!

[codecov-io]

Codecov Report

Merging #631 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
+ Coverage   86.46%   86.52%   +0.05%     
==========================================
  Files          91       91              
  Lines       20626    20628       +2     
==========================================
+ Hits        17834    17848      +14     
+ Misses       2792     2780      -12

Impacted Files	Coverage Δ
src/router_core/delivery.c	89.37% <100%> (+0.19%)	⬆️
src/router_core/forwarder.c	93.95% <100%> (ø)	⬆️
src/router_core/connections.c	94.43% <100%> (+0.56%)	⬆️
src/router_core/transfer.c	94.84% <100%> (+1.47%)	⬆️
src/iterator.c	89.34% <0%> (-0.17%)	⬇️
src/router_node.c	93.09% <0%> (-0.12%)	⬇️
src/container.c	81.37% <0%> (+0.18%)	⬆️
src/parse.c	87.86% <0%> (+0.22%)	⬆️
... and 1 more

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