[leofs-adm][docs] provide operations to delete queue files safely

[mocchira]

Consider to add the description how to delete queue files manually into the doc. reference: https://gist.github.com/mocchira/1c4852c57c7b328aef46eb234b74093b

Also think of providing this feature through leofs-adm for safe with as less human operations as possible.

Note that we also have to warn the risk of deleting queue files that may cause inconsistencies of objects across a cluster due to being lost sync information included in queues.