provider: persist provide queue on shutdown #1160
Description
Follow up to #1159
To allow consistent provide operations, the provide queue should be persisted on shutdown to allow resuming provides after a restart.
This is necessary for keys in the provide queue, not belonging to a region that is late for reprovide at the time of the restart. Not persisting the queue implies that keys in the provide queue would need to wait up to reprovideInterval - downTime before being provided for the first time.
If a queued key belong to a region that is late for reprovide on restart, then it will be provided as part of the region reprovide, which will happen ASAP.