CSVBatch.java

public class CSVBatch implements Database.Batchable<PolicyMaster__c>, Database.AllowsCallouts, Database.Stateful {
	private CSVState state;
	private Integer scopeSize;

	public CSVBatch (CSVState state) {
		this.state = state;
	}

	public Database.QueryLocator start (Database.BatchableContext ctx) {
		return Database.getQueryLocator([
			SELECT Id
			FROM PolicyMaster__c
			WHERE PolicyNumber__c IN :state.getPolicyNumbers()]);
	}

	public void execute (Database.BatchableContext ctx, List<PolicyMaster__c> scope) {
		// for each record in scope
		// add state.policies.get(scope.PolicyNumber__c) to scope
		// upsert scope
		// add errors to state.errors
	}

	public void finish (Database.BatchableContext ctx) {
		if (state.hasNext()) {
			System.enqueueJob(new CSVQueueable(state));

		} else {
			// insert errors
			// send email to admins
		}
	}
}

CSVQueueable.java

public class CSVQueueable implements Queueable, Database.AllowsCallouts {
	private CSVState state;
	private Integer csvChunkSize;

	public CSVQueueable (CSVState state) {
		this(state, state.getChunkSize());
	}

	public CSVQueueable (Blob csvData, Integer chunkSize) {
		this.state = new CSVState(csvData, chunkSize);
		this.csvChunkSize = chunkSize;
	}

	public CSVQueueable(CSVState state, Integer chunkSize) {
		this.state = state;
		this.csvChunkSize = chunkSize;
	}

	public void execute (QueueableContext ctx) {
		state.next();
		Database.executeBatch(new CSVBatch(state));
	}
}

CSVState.java

public with sharing class CSVState {
	// Policies currently in processing
	private Map<String, PolicyMaster__c> policies;
	private List<CSVRow> rows;
	private Integer chunkSize;

	// All Policy Numbers processed
	private Set<String> processedPolicyNumbers;
	// Any errors encountered during batch execution
	private List<Error> errors;

	public CSVState(Blob csvData, Integer size) {
		this.errors = new List<Error>();
		this.chunkSize = size;
		parseCSV(csvData);
	}

	public Integer getChunkSize() {
		return this.chunkSize;
	}

	public void next() {
		// Get the Next [chunkSize] Unique Policy Numbers from this.rows
		// Replay CSVRows in chronological order into PolicyMaster__c records.
		// this.policies = chronological, aggregated PolicyMaster__c changes in this.rows for [chunksize]
	}

	public Boolean hasNext() {
		// has more policyNumbers to process
	}

	public Set<String> getPolicyNumbers() {
		// All Policy Numbers in current chunk
		return this.policies.keySet();
	}

	public void addError (String policyNumber, Exception exc) {
		// add error to this.errors
	}

	private void parseCSV(Blob csvData) {
		// parse csv data into List<CSVRow>
		this.rows = new List<CSVRow>();
	}

	// Parse CSV Blob Data to List of Rows
	public class CSVRow {
		public String PolicyNumber;
	}

	public class Error {
		public String policyNumber;
		public String errorMessage;
	}
}