Spike for parallel relaying between multiple channels

[romac]

Closes: #XXX

Description

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For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[codecov-io]

Codecov Report

Merging #692 (317f3d7) into master (b1b37f5) will increase coverage by 30.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    informalsystems/hermes#692      +/-   ##
=========================================
+ Coverage    13.6%   43.8%   +30.1%     
=========================================
  Files          69     154      +85     
  Lines        3752   10122    +6370     
  Branches     1374       0    -1374     
=========================================
+ Hits          513    4438    +3925     
- Misses       2618    5684    +3066     
+ Partials      621       0     -621     

Impacted Files	Coverage Δ
modules/src/address.rs	100.0% <ø> (ø)
...application/ics20_fungible_token_transfer/error.rs	0.0% <ø> (ø)
...ion/ics20_fungible_token_transfer/msgs/transfer.rs	0.0% <ø> (ø)
modules/src/events.rs	0.0% <ø> (ø)
modules/src/handler.rs	100.0% <ø> (ø)
modules/src/ics02_client/client_def.rs	47.7% <ø> (ø)
modules/src/ics02_client/client_type.rs	79.1% <ø> (+31.5%)	⬆️
modules/src/ics02_client/context.rs	100.0% <ø> (ø)
modules/src/ics02_client/error.rs	100.0% <ø> (ø)
modules/src/ics02_client/events.rs	15.7% <ø> (+15.7%)	⬆️
... and 257 more

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[ancazamfir]

This looks amazing! I ran it with three different channels and all looks good. Also reviewed the code, I think we should merge this asap, can't wait to have this in master :)
Thanks Romain!