move tbwatcher to handler thread (add interface to workers)

[raubitsj]

I had put tbwatcher into the sender thread because it was easier before. this is more correct.

sender doesnt run during offline mode.

this also ends up cleaning up some of the tensorboard image hacks by reusing interface.publish_history() rather than reimplementing the data_type json conversion in a different place.

Testing regression: https://app.wandb.ai/jeffr/regression/groups/20200824-0.0.41-fix_tbwatcher_to_handler-c05ffe6-im7ij9

[coveralls]

Pull Request Test Coverage Report for Build 2e7da36c-a572-41c1-ae3d-61be60cd53ad

• 60 of 89 (67.42%) changed or added relevant lines in 10 files are covered.
• 16 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.06%) to 60.483%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wandb/internal/stats.py	1	2	50.0%
wandb/internal/sender.py	22	24	91.67%
wandb/internal/internal.py	3	6	50.0%
wandb/internal/tb_watcher.py	2	8	25.0%
wandb/internal/handler.py	11	28	39.29%
<!--	Total:	60	89	67.42%	-->

Files with Coverage Reduction	New Missed Lines	%
wandb/integration/tensorboard/log.py	1	64.81%
wandb/internal/handler.py	1	44.92%
wandb/internal/internal.py	2	27.89%
wandb/internal/run.py	3	61.54%
wandb/internal/tb_watcher.py	9	21.16%
<!--	Total:	16		-->

Totals
Change from base Build 539a585d-edc3-4fda-ba24-ae61de0aa9f9:	-0.06%
Covered Lines:	12030
Relevant Lines:	19890

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💛 - Coveralls

[adrnswanberg]

nice!