Move more code under mpool feature

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.58%. Comparing base (ec8eb23) to head (d5ced80).

Files	Patch %	Lines
ntex-bytes/src/pool.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #342      +/-   ##
==========================================
- Coverage   90.62%   90.58%   -0.04%     
==========================================
  Files         197      197              
  Lines       31634    31642       +8     
==========================================
- Hits        28669    28664       -5     
- Misses       2965     2978      +13     

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