Refactor sending to do less retrying and let handlers be dumber

[rowanseymour]

See #653

• Allows handlers to implement one of two interfaces that differ by send method to allow handlers to be rewritten piece meal.
• New Send contract takes away need for handlers creating StatusUpdate objects - they return different error singletons and then sender determines whether message should be be retried.
• Or for things like db errors.. they can return a regular error and that is treated as a logged, retryable internal error
• Have send errors become channel log errors to avoid duplicating those.

[codecov[bot]]

Codecov Report

Attention: 78 lines in your changes are missing coverage. Please review.

Comparison is base (bf0489f) 74.53% compared to head (6f13343) 74.30%.

:exclamation: Current head 6f13343 differs from pull request most recent head 59edbea. Consider uploading reports for the commit 59edbea to get more accurate results

Files	Patch %	Lines
sender.go	32.72%	34 Missing and 3 partials :warning:
handlers/test.go	0.00%	29 Missing :warning:
handlers/africastalking/handler.go	50.00%	4 Missing and 2 partials :warning:
handlers/arabiacell/handler.go	72.72%	2 Missing and 1 partial :warning:
handlers/tembachat/handler.go	66.66%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #687 +/- ## ========================================== - Coverage 74.53% 74.30% -0.23% ========================================== Files 98 98 Lines 13046 13093 +47 ========================================== + Hits 9724 9729 +5 - Misses 2635 2677 +42 Partials 687 687 ```

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