Move shutdownfns, terminatefns and hammerfns out of separate goroutines

[zeripath]

Coalesce the shutdownfns etc into a list of functions that get run at shutdown
rather then have them run at goroutines blocked on selects.

This may help reduce the background select/poll load in certain
configurations.

Signed-off-by: Andrew Thornton art27@cantab.net

[zeripath]

Ref #15048
Ref #15542

[codecov-commenter]

Codecov Report

Merging #15686 (8d3464a) into master (8c8471e) will decrease coverage by 0.02%.
The diff coverage is 22.36%.

@@            Coverage Diff             @@
##           master   #15686      +/-   ##
==========================================
- Coverage   43.90%   43.88%   -0.03%     
==========================================
  Files         678      678              
  Lines       81868    81883      +15     
==========================================
- Hits        35946    35934      -12     
- Misses      40075    40096      +21     
- Partials     5847     5853       +6     

Impacted Files	Coverage Δ
modules/graceful/manager.go	25.56% <15.94%> (-3.47%)	⬇️
modules/graceful/context.go	18.18% <66.66%> (-46.11%)	⬇️
modules/graceful/manager_unix.go	36.63% <100.00%> (ø)
modules/charset/charset.go	71.71% <0.00%> (-4.05%)	⬇️
modules/notification/mail/mail.go	41.83% <0.00%> (+3.06%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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[zeripath]

I'm gonna close this to move its changes in to #15693