Clean up stage exit handling

[mvines]

• Uniformly pass the exit signal as a reference, defer cloning until actually needed (ie, a new thread is spawned)
• Remove a lot of the exit()'/close()` function copy pasta. Most stages don't need 2-3 ways to exit
• Scrapped away a bit of dead code as a result of the previous two items

[codecov]

Codecov Report

Merging #3122 into master will increase coverage by <.1%.
The diff coverage is 89.8%.

@@           Coverage Diff            @@
##           master   #3122     +/-   ##
========================================
+ Coverage    78.7%   78.7%   +<.1%     
========================================
  Files         134     134             
  Lines       20075   19942    -133     
========================================
- Hits        15800   15704     -96     
+ Misses       4275    4238     -37

[codecov]

Codecov Report

Merging #3122 into master will increase coverage by <.1%.
The diff coverage is 89.8%.

@@           Coverage Diff            @@
##           master   #3122     +/-   ##
========================================
+ Coverage    78.7%   78.7%   +<.1%     
========================================
  Files         134     134             
  Lines       20075   19942    -133     
========================================
- Hits        15800   15704     -96     
+ Misses       4275    4238     -37