Copy backoff module from Trillian

[gdbelvin]

A little copying is better than a little dependency

• Go Proverb

[codecov]

Codecov Report

Merging #1424 into master will decrease coverage by 0.01%.
The diff coverage is 82.5%.

@@            Coverage Diff             @@
##           master    #1424      +/-   ##
==========================================
- Coverage   66.33%   66.31%   -0.02%     
==========================================
  Files          53       54       +1     
  Lines        3986     4026      +40     
==========================================
+ Hits         2644     2670      +26     
- Misses        952      962      +10     
- Partials      390      394       +4

Impacted Files	Coverage Δ
internal/backoff/backoff.go	82.5% <82.5%> (ø)
core/sequencer/trillian_client.go	58.57% <0%> (-4.29%)	⬇️
impl/sql/directory/storage.go	67.66% <0%> (-1.51%)	⬇️
core/sequencer/server.go	72.96% <0%> (-0.66%)	⬇️

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[pav-kv]

LGTM % file location and spelling warning.

[NatalieDoduc]

Is there any way of denoting that this is a copy of code that is shared in Trillian (and vice versa)? I notelic lid your copy vs dependency proverb, but the potential to divergence in both flavours is worrisome.

While here - was there not a public backoff or retry library that was suitable?

[gdbelvin]

Divergence is less of a risk here because we can fully test it in this project without worrying about breaking other projects. Perhaps I'm missing something though.

grpc-go has a backoff library but it is also inside an internal directory.