pickfirstleaf: minor simplification to reconcileSubConnsLocked method

[easwars]

This PR adds a minor simplification to the reconcileSubConnsLocked method

• Instead of building a new map for the existing subchannels with the key being the address and the value being a boolean (which is always set to true), we can instead use the existing map.

This PR also adds a docstring to the method instead of inline comments, some of which were a little stale.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.99%. Comparing base (b850ea5) to head (9e3a8f3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7731      +/-   ##
==========================================
+ Coverage   81.96%   81.99%   +0.02%     
==========================================
  Files         362      362              
  Lines       28119    28116       -3     
==========================================
+ Hits        23048    23053       +5     
+ Misses       3863     3858       -5     
+ Partials     1208     1205       -3     

Files with missing lines	Coverage Δ
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	88.88% <100.00%> (-0.13%)	⬇️

... and 15 files with indirect coverage changes

[arjan-bal]

LGTM, thanks!