(Re)Starting a Data Node Holding a Large Number of Indices can Take Minutes

[original-brownbear]

Restarting a data node holding lots of indices takes an unexpected amount of time.
For a node holding ~10k Beats mapping indices, it takes ~10 minutes to re-start the node.

Effectively all the time is spent on validating mappings on a single hot thread:

We should find a way to speed this up, adding a 10 minute window to restarting a large data node makes operations complicated.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[DaveCTurner]

I think we could skip this if it's not an upgrade? Absent awful bugs, this should be a no-op if the starting node is the same version as the one that last wrote the metadata, and we include the writing node's version in the commit metadata so could plumb it through via PersistedClusterStateService#OnDiskState.

[DaveCTurner]

Given #84148, we don't really think validating these mappings at startup is useful anyway and therefore we could just skip this validation always. Or do nothing and wait for #84148 to be addressed.