Add option to random load balance to >1 server nodes

[oarap]

Add option to random load balance to >1 server nodes

Adding possibility to set peerConnectionCount so that a single client
node can do a random load balancing among peerConnectionCount top
nodes using the score calculator and peer heap.

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[codecov]

Codecov Report

Merging #766 into dev will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #766      +/-   ##
==========================================
- Coverage   88.22%   88.09%   -0.14%     
==========================================
  Files          40       40              
  Lines        4101     4115      +14     
==========================================
+ Hits         3618     3625       +7     
- Misses        367      374       +7     
  Partials      116      116              

Impacted Files	Coverage Δ
peer.go	93.77% <100.00%> (-0.42%)	⬇️
mex.go	72.51% <0.00%> (-1.90%)	⬇️
reqres.go	81.19% <0.00%> (-1.71%)	⬇️
outbound.go	85.96% <0.00%> (-1.17%)	⬇️
channel.go	88.46% <0.00%> (-0.28%)	⬇️
relay.go	82.87% <0.00%> (ø)
preinit_connection.go	94.16% <0.00%> (+1.45%)	⬆️
idle_sweep.go	100.00% <0.00%> (+3.92%)	⬆️

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[prashantv]

This looks very similar to #631 -- can we consolidate into a single PR? Ideally into the already existing PR, as it's got some history + comments.