fix: Revert "Use thread pools (#601)"

[barbeau]

This reverts commit 6619f14 (and I added comments to avoid someone using a thread pool again in the future).

It seems that when a thread pool is used it allows a thread to try and unlock() a lock that it did not lock(), which ReentrantLock does not allow - "If the current thread is not the holder of this lock then IllegalMonitorStateException is thrown."

I can't reproduce #660 myself, but based on the description and stack trace in #660 it makes sense that this change could cause this issue in certain multi-threaded states.

It would be good to get a release out in the near term with this fix to see if it does indeed resolve the issue.

Fixes #660

[codecov]

Codecov Report

Merging #665 into master will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #665      +/-   ##
==========================================
+ Coverage   39.33%   39.35%   +0.01%     
==========================================
  Files          71       71              
  Lines        4037     4035       -2     
  Branches      606      606              
==========================================
  Hits         1588     1588              
+ Misses       2346     2344       -2     
  Partials      103      103              

Impacted Files	Coverage Δ
.../clustering/algo/PreCachingAlgorithmDecorator.java	0.00% <0.00%> (ø)
...ndroid/clustering/view/DefaultClusterRenderer.java	0.00% <0.00%> (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 8689d29...3131b18. Read the comment docs.

[arriolac]

LGTM