Tweak logging. Add timezone, use everywhere.

[lkysow]

• Use logger for all warnings. Previously we were writing warnings from
  cmd/server.go directly to stderr which bypassed our normal log format.
• Add the timezone to log output. This is just really nice to have if
  you're looking at old logs.
• Change levels to all be 4 characters: DBUG, INFO, WARN, EROR. This
  makes the logs easier to read because it lines up.
• Log when we first receive the request as well as when we send a
  response. This makes it easier to see where the request starts and ends.

[codecov]

Codecov Report

Merging #402 into master will increase coverage by 0.09%.
The diff coverage is 65.27%.

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
+ Coverage   69.84%   69.94%   +0.09%     
==========================================
  Files          62       63       +1     
  Lines        3817     3849      +32     
==========================================
+ Hits         2666     2692      +26     
- Misses        961      963       +2     
- Partials      190      194       +4

Impacted Files	Coverage Δ
server/middleware.go	0% <0%> (ø)	⬆️
main.go	0% <0%> (ø)	⬆️
server/user_config.go	100% <100%> (ø)
cmd/server.go	78.23% <11.11%> (-0.94%)	⬇️
server/server.go	66.27% <50%> (ø)	⬆️
server/logging/simple_logger.go	88.75% <78.12%> (+4.46%)	⬆️
server/events/command_runner.go	70.71% <81.81%> (-0.61%)	⬇️

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