Change logging output to os.Stderr

[gm42]

Which problem is this PR solving?

It is not safe to write directly to os.Stdout/os.Stderr in concurrent scenarios; writes can intermingle and cause corrupt log lines output. See also: https://stackoverflow.com/a/14694666

It turns out it is safe instead, see:

• https://github.com/golang/go/blob/go1.14.3/src/internal/poll/fd_unix.go#L255
• https://github.com/golang/go/blob/go1.14.3/src/internal/poll/fd_windows.go#L684

(No issue created yet)

Short description of the changes

Switch logging output to os.Stderr; this is the usual file descriptor for log output, also used by Go's log package: https://golang.org/src/log/log.go?s=9636:9664#L76

[codecov]

Codecov Report

Merging #213 into master will decrease coverage by 10.26%.
The diff coverage is 51.93%.

@@             Coverage Diff             @@
##           master     #213       +/-   ##
===========================================
- Coverage   84.48%   74.22%   -10.27%     
===========================================
  Files          52       56        +4     
  Lines        3074     3212      +138     
===========================================
- Hits         2597     2384      -213     
- Misses        398      764      +366     
+ Partials       79       64       -15     

Impacted Files	Coverage Δ
client/amqp/amqp.go	40.98% <ø> (ø)
client/amqp/option.go	66.66% <ø> (ø)
client/amqp/user.pb.go	26.92% <ø> (ø)
client/http/option.go	100.00% <ø> (ø)
client/kafka/async_producer.go	0.00% <0.00%> (ø)
client/kafka/sync_producer.go	0.00% <0.00%> (ø)
client/redis/redis.go	44.44% <ø> (ø)
client/sns/message.go	97.53% <ø> (ø)
client/sns/publisher.go	93.33% <ø> (ø)
client/sql/sql.go	17.53% <ø> (-68.73%)	⬇️
... and 71 more

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