Update to Go 1.12.5 testing. Remove deprecated unused and gosimple
pacakges, and mask staticcheck 1006. Also, fix unconvert errors related
to unnecessary type conversions and following staticcheck errors:
- remove redundant return statements
- use for range instead of for select
- use time.Since instead of time.Now().Sub
- omit comparison to bool constant
- replace T.Fatal and T.Fatalf in tests with T.Error and T.Fatalf respectively because the goroutine calls T.Fatal must be called in the same goroutine as the test
- fix error strings that should not be capitalized
- use sort.Strings(...) instead of sort.Sort(sort.StringSlice(...))
- use he status code of Canceled instead of grpc.ErrClientConnClosing which is deprecated
- use use status.Errorf instead of grpc.Errorf which is deprecated
Related #10528#10438
clientv3 integration test was using clientv3.NewKV, clientv3.NewWatcher, etc to create specific client.
replace those with direct client calls so that the direct calls can also test grpc proxy.
Causes async reconnect to block while the client is dialing.
This was also causing problems with the Close error message, so
now Close() will return the last dial error (if any) instead of
clearing it out with a cancel().
Fixes#5416
