The field schema incorrectly stores serial fields as int

Problem/Motivation

This was discovered in #2841291: Fix NOT NULL handling in the entity storage and 'primary key' changes when updating the storage definition of an identifier field.

The field schema data doesn't accurately store the schema for identifier fields (ID and Revision ID): Fields that are stored as serial in certain tables are actually saved as int in the field schema data for those tables.

Proposed resolution

Move the ::processIdentifierSchema() calls from process*Table() to getSharedTableFieldSchema().