grpclb: update picker synchronously on config update

[aranjans]

#5469 recommends an audit of existing LB policies to ensure that they update their pickers synchronously upon receipt of a configuration update. grpclb does not update picker synchronously.

What does this PR do?
This PR ensures that every time configuration update is triggered, picker is updated synchronously.

RELEASE NOTES:

• grpclb: update picker synchronously on receipt of configuration update.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.90%. Comparing base (3eb0145) to head (68cebc6).
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7628      +/-   ##
==========================================
+ Coverage   81.57%   81.90%   +0.32%     
==========================================
  Files         354      361       +7     
  Lines       27076    27823     +747     
==========================================
+ Hits        22088    22788     +700     
- Misses       3798     3844      +46     
- Partials     1190     1191       +1     

Files with missing lines	Coverage Δ
balancer/grpclb/grpclb.go	83.49% <100.00%> (+0.81%)	⬆️

... and 91 files with indirect coverage changes