capture write times to adjust lease timeout

[callmehiphop]

Fixes #83
Fixes #88

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Summary of Changes

Currently we "auto-lease" messages for the user, this means that we continuously modify the ackdeadline in an attempt to prevent messages being processed from being re-delivered. This involves setting a recursive timeout which is calculated like so

99th percentile of ack times * 0.9 * random (jitter)

However this doesn't account for any latency from writing to the subscription stream. This PR captures the time it takes to finish writing to a stream and adjusts the lease timeout accordingly.

[codecov]

Codecov Report

Merging #99 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #99   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      8           
  Lines         863    869    +6     
=====================================
+ Hits          863    869    +6

Impacted Files	Coverage Δ
src/subscription.js	100% <100%> (ø)	⬆️
src/histogram.js	100% <100%> (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 77b2d74...bf24796. Read the comment docs.

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.