Only use boost workers for leveldb shadow queues (#15696)

Only use boost workers for leveldb shadow queues (#15696)

* 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.

* create a zero boost so that if there are no workers in a pool - boost to start the workers

* actually set timeout appropriately on boosted workers

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