Only use boost workers for leveldb shadow queues

[zeripath]

The leveldb shadow queue of a persistable channel queue should always start with 0
workers and just use boost to add additional workers if necessary.

Fixes #15542

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

[codecov-commenter]

Codecov Report

Merging #15696 (a6682bd) into master (6ebd833) will decrease coverage by 0.01%.
The diff coverage is 71.05%.

@@            Coverage Diff             @@
##           master   #15696      +/-   ##
==========================================
- Coverage   43.91%   43.89%   -0.02%     
==========================================
  Files         678      678              
  Lines       81838    81866      +28     
==========================================
  Hits        35936    35936              
- Misses      40052    40083      +31     
+ Partials     5850     5847       -3     

Impacted Files	Coverage Δ
modules/queue/workerpool.go	51.64% <63.33%> (-7.13%)	⬇️
modules/queue/queue_disk_channel.go	70.77% <100.00%> (ø)
modules/queue/unique_queue_disk_channel.go	55.22% <100.00%> (ø)
services/pull/pull.go	43.47% <0.00%> (ø)
models/gpg_key.go	54.07% <0.00%> (+0.56%)	⬆️

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[zeripath]

This also fixes the timeout times reported on the process manager for boost workers and also makes it possible to start all queues with zero workers to allow defaulting to boost when they get some work.