Throttle send-msg for IRC to prevent flooding.

[sarg]

Allow 1 request per second with burstiness up to 5.
Use log/warn when server reports flooding.

Seems that 1 rps is the default setting for IRC protocol. I've played with different throttling strategies on freenode using !range 20 | xargs echo and seems that current one is pretty safe.

[codecov]

Codecov Report

Merging #111 into master will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   43.76%   43.73%   -0.04%     
==========================================
  Files          90       90              
  Lines        3578     3583       +5     
  Branches      166      166              
==========================================
+ Hits         1566     1567       +1     
- Misses       1846     1850       +4     
  Partials      166      166

Impacted Files	Coverage Δ
src/yetibot/core/adapters/irc.clj	24.3% <33.33%> (-0.13%)	⬇️

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[devth]

@sarg this looks great! Will test it out later today.

[devth]

Played with this, it's awesome. I didn't find any official docs on allows rps on freenode but I think 1 rps is pretty safe, though it is a bit slow. Might be something we make configurable.

Thanks for the PR!