Handle database replication delay when adding todos

[gabrieljackson]

This change adds logic to ensure newly-added todos are in the
returned list. When the Mattermost server running the plugin has
database read replicas configured, it is possible that the plugin
would miss listing the new todo due to replication delay. This
plugin logic attempts to address the issue when it is detected.

I verified this issue and fix by using a dev server with a database
with read replicas configured.

When writing a fix, I went with something that felt like a good
trade-off on code complexity and speed. i.e. I avoided a simple
sleep, but also didn't want to drastically refactor code to fix
the issue in a slightly-better way. All that said, let me know what
you think.

Fixes #87

[codecov-commenter]

Codecov Report

Merging #93 into master will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master     #93   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           8       8           
  Lines         858     868   +10     
======================================
- Misses        858     868   +10     

Impacted Files	Coverage Δ
server/command.go	0.00% <0.00%> (ø)
server/list.go	0.00% <0.00%> (ø)
server/plugin.go	0.00% <0.00%> (ø)

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[larkox]

LGTM! Simple and elegant.

Thank you very much for the fix 😄

[mickmister]

LGTM!

[mickmister]

@DHaussermann You may not be able to reproduce the initial issue due to needing the db replica setup. I think this will just need a general smoke test.

[gabrieljackson]

Yeah, even once I had a proper test environment, the issue took many /todo add commands to manifest itself. It definitely is tricky to find and verify outside of something like our community server.

[gabrieljackson]

Just checking that this isn't waiting on me for anything.

cc. @DHaussermann

[DHaussermann]

Tested and passed
Regression tested the plugin for proper functionality

• Various cases of creating and sending ToDos
• Creating from a post and adding to thread are working
• All DMs form ToDo bot based on other user actions are working as expected
• Slash commands are working
  LGTM!