Today we reroute the cluster as part of the process of starting a shard, which
runs at `URGENT` priority. In large clusters, rerouting may take some time to
complete, and this means that a mere trickle of shard-started events can cause
starvation for other, lower-priority, tasks that are pending on the master.
However, it isn't really necessary to perform a reroute when starting a shard,
as long as one occurs eventually. This commit removes the inline reroute from
the process of starting a shard and replaces it with a deferred one that runs
at `NORMAL` priority, avoiding starvation of higher-priority tasks.
Backport of #44433 and #44543.
452f7f67a0