introspection: Add inactive connections to output

[prashantv]

Tried to debug a stuck connection that was stuck in "startClose", but
unfortunately the introspection output was missing the connection as it
expects connections to be added as part of root peers, but peers only
store references to active connections.

Output inactive connections at the top level of the channel (which is
the only thing that has references to inactive connections).

[codecov]

Codecov Report

Merging #686 into dev will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #686      +/-   ##
==========================================
+ Coverage   86.68%   86.92%   +0.24%     
==========================================
  Files          39       39              
  Lines        3959     3963       +4     
==========================================
+ Hits         3432     3445      +13     
+ Misses        403      399       -4     
+ Partials      124      119       -5

Impacted Files	Coverage Δ
introspection.go	91.3% <100%> (+0.17%)	⬆️
mex.go	72.09% <0%> (-7.91%)	⬇️
outbound.go	78.73% <0%> (-2.88%)	⬇️
relay.go	84.53% <0%> (+0.65%)	⬆️
channel.go	88.03% <0%> (+1.42%)	⬆️
preinit_connection.go	94.16% <0%> (+1.45%)	⬆️
root_peer_list.go	96% <0%> (+4%)	⬆️
connection.go	84.63% <0%> (+5.03%)	⬆️

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