JsonFileWriter fails with NPE after VectorSchemaRoot syncSchema

Hey there :)

Repro:
- Open VSR
- Do something that'll change the `Field` of one of the vectors in the VSR (promotable writer, etc)
- `vsr.syncSchema()`
- Open JsonFileWriter, `start`, `write` -> NPE

Looks like the cause is that the `fieldVectorsMap` cache isn't updated in `syncSchema`? Possibly a few other places - `addVector`/`removeVector` too?

Cheers,

James

**Reporter**: [James Henderson](https://issues.apache.org/jira/browse/ARROW-11385) / @jarohen

_{**Note**: *This issue was originally created as [ARROW-11385](https://issues.apache.org/jira/browse/ARROW-11385). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Riproduci la sequenza che coinvolge VectorSchemaRoot, syncSchema e JsonFileWriter menzionata nel report, quindi esamina la cache fieldVectorsMap e i percorsi addVector/removeVector menzionati nel report. Il lavoro è completato quando JsonFileWriter può avviarsi e scrivere dopo la modifica di un Field vettoriale e syncSchema termina senza una NPE.

Scritto dal modello di indicizzazione a partire dal testo della issue.