VectorSchemaRoot loses columns with equal Fields in JSON files, and syncSchema() drops the schema metadata #1473

Describe the bug, including details regarding any error messages, version, and platform.

Two problems in VectorSchemaRoot, found while reviewing #1389:

• VectorSchemaRoot keys its vector lookup by Field, and Field equality is structural, so columns with equal Fields (for example, two int columns both named x) share one entry. That entry holds the last of the columns, while getVector(String) documents that it returns the first. JsonFileWriter and JsonFileReader look each column up through getVector(Field), so they write and read the same vector for both columns: a JSON round trip of x: [1], x: [2] reads back x: [null], x: [2].
• syncSchema() rebuilds the schema with new Schema(newFields), which drops the custom metadata of the schema.

Both problems predate #1389.