Fix issue in connection pool handle queue behavior; add tests around …

[DenverM80]

…concurrent transfers of files of various sizes

cherry-picked all the commits from #207 and then squashed together into a single commit.

Merging to a branch in order to create a 3.5.3 release

==13536== LEAK SUMMARY:
==13536==    definitely lost: 0 bytes in 0 blocks
==13536==    indirectly lost: 0 bytes in 0 blocks
==13536==      possibly lost: 0 bytes in 0 blocks
==13536==    still reachable: 20,967 bytes in 29 blocks
==13536==         suppressed: 0 bytes in 0 blocks
==13536== 
==13536== For counts of detected and suppressed errors, rerun with: -v
==13536== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
[100%] Built target mem