allow updating of doc mapper

[trinity-1686a]

Description

this pr aims at laying the foundation work for updating the doc mapping of an index

• store a doc mapper version in the doc mapper
• store a doc mapper version in splits meta
• allow updating the doc mapper, auto-incrementing its version (metastore level)
• allow updating the doc mapper (api level)
• block merges between multiple version of a doc mapper
• restart ingestion pipeline so they notice the change in doc mapper decently quickly (stretch goal, maybe put in a 2nd pull request)

How was this PR tested?

• test we properly block merges (unit tests)
• test we properly block merges (integration tests)
• test modifying different properties (tokenizer, what is stored...), and how it breaks (or not) search

[rdettai]

We should probably merge #4868 before addressing:

• allow updating the doc mapper (api level)