optimize fast path with Duration-based wait time

[jeremyandrews]

• reverted Allow to set duration instead of seconds for task wait configuration #358
• re-implemented with minimal changes to the underlying logic
• optimized the fast path avoiding unnecessary Duration->integer conversions
• fixes Throughput performance drop #367 -- the issue was the amount of time it take to convert Duration->integer, while small, adds up in the fast path where this can be happening tens of thousands of times a second: the code is optimized to not perform any conversions if there is no wait time set

0.13.3

• 9,668
• 10,385
• 10,031

0.14.1-dev (before this PR)

• 7,526
• 7,739
• 7,669

0.14.1-dev (after this PR)

• 9,919
• 10,206
• 10,082

[jeremyandrews]

I've run a comparison of 0.13.3, 0.14.1-dev without this PR, and 0.14.1-dev with this PR, confirming that this seems to fix the reported performance regression:
https://docs.google.com/spreadsheets/d/1E7LwL_kJmRgRJ_BVW3QPTIoe7hqmODGhOpiGWjppuNY/edit#gid=0

[zezic]

I can confirm that this PR fixes the #367.

[LionsAd]

RTBM - with two nits / questions