balancer/wrr: prefer calling Equal() method of time.Time

[aranjans]

Prefer calling Equal() method of time.Time over direct comparision.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.83%. Comparing base (b45fc41) to head (359462b).
Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7529      +/-   ##
==========================================
- Coverage   82.07%   81.83%   -0.25%     
==========================================
  Files         360      360              
  Lines       27533    27508      -25     
==========================================
- Hits        22599    22510      -89     
- Misses       3759     3799      +40     
- Partials     1175     1199      +24     

Files	Coverage Δ
balancer/weightedroundrobin/balancer.go	80.51% <100.00%> (ø)

... and 29 files with indirect coverage changes

[easwars]

There are a few more occurrences:

easwars@minetop: grpc-go$ grep -r "==.*time.Time{}" *
balancer/weightedroundrobin/balancer.go:	if w.nonEmptySince == (time.Time{}) {
balancer/weightedroundrobin/balancer.go:	if w.lastUpdated == (time.Time{}) {
balancer/weightedroundrobin/balancer.go:	if blackoutPeriod != 0 && (w.nonEmptySince == (time.Time{}) || now.Sub(w.nonEmptySince) < blackoutPeriod) {
test/config_selector_test.go:			if wantDeadline == (time.Time{}) {