Synonyms API - apply synonym updates sequentially

[carlosdelest]

Synonyms API rely on synonyms being stored on a system index. Each synonym is stored as a separate document on that index.

Retrieving and updating synonyms means doing bulk updates and searching on the index. This means that applying concurrent updates can result in inconsistent status for synonyms.

Synonyms should be updated as a master operation to ensure they are applied sequentially.

See #109390 for details.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)