Replace flume with async-channel

[sagebind]

Flume changes its minimum Rust version too often and also recently added a dependency chain involving longer compile times than ideal. Async-channel is comparable to flume for our use-case, is stable, and has a smaller dependency tree.

[codecov]

Codecov Report

Merging #318 (f2405ac) into master (0c47d7c) will not change coverage.
The diff coverage is 76.92%.

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   75.07%   75.07%           
=======================================
  Files          57       57           
  Lines        2993     2993           
=======================================
  Hits         2247     2247           
  Misses        746      746           

Impacted Files	Coverage Δ
src/handler.rs	71.10% <66.66%> (ø)
src/agent/mod.rs	72.68% <80.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0c47d7c...f2405ac. Read the comment docs.