Use Base.alloc_request instead of implementing it directly

[vtjnash]

There is slightly better error handling in this function, which avoids relying as directly on implementation details of IOBuffer

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.67%. Comparing base (040cf0a) to head (90f3d79).
⚠️ Report is 48 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1145      +/-   ##
==========================================
- Coverage   82.67%   82.67%   -0.01%     
==========================================
  Files          32       32              
  Lines        3054     3053       -1     
==========================================
- Hits         2525     2524       -1     
  Misses        529      529              

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