Darren/enhancement/use slog

[darrenvechain]

Description

Use the built in slog package. log package copied from latest geth. Also introduced a new flag --json-logs which will output the logs in JSON format.

Type of change

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• [x] log package unit tests, from ethereum.
• [x] manually

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] New and existing E2E tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.57962% with 392 lines in your changes missing coverage. Please review.

Project coverage is 62.14%. Comparing base (830b1a6) to head (117922f). Report is 2 commits behind head on master.

Files	Patch %	Lines
log/format.go	62.89%	67 Missing and 15 partials :warning:
log/logger.go	26.43%	62 Missing and 2 partials :warning:
log/handler.go	48.93%	46 Missing and 2 partials :warning:
log/root.go	4.08%	47 Missing :warning:
cmd/thor/utils.go	0.00%	35 Missing :warning:
cmd/disco/utils.go	0.00%	26 Missing :warning:
cmd/thor/node/node.go	0.00%	26 Missing :warning:
cmd/thor/main.go	0.00%	21 Missing :warning:
cmd/thor/node/packer_loop.go	0.00%	12 Missing :warning:
p2psrv/server.go	0.00%	7 Missing :warning:
... and 11 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #781 +/- ## ========================================== - Coverage 62.59% 62.14% -0.46% ========================================== Files 199 203 +4 Lines 18165 18688 +523 ========================================== + Hits 11371 11613 +242 - Misses 5713 5974 +261 - Partials 1081 1101 +20 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.