Refactor PDF exports for headless printing

[sixlettervariables]

This PR refactors PDF exports to more easily allow headless printing.

Here is an example of it in use from a web service:

    private byte[] createRecordSheet(LambdaLogger logger, Entity entity) {
        try {
            return new RecordSheetBuilder().setEntity(entity).buildPdf();
        } catch (Exception e) {
            // ...
        }
    }

    @Override
    protected void handleRequest(...) {
        // ...
        String category = getParameter(event, "category");
        String unitPath = getParameter(event, "unit");
        String path = String.format("%s/%s", category, unitPath);

        Entity entity;
        try {
            entity = s3.readEntity(importBucket, path);
        } catch (EntityLoadingException | SdkException e) {
            // ...
        }

        // ...

        Util.ensureHeadless();
        Util.ensureLocalFonts(logger);

        byte[] pdfBytes = createRecordSheet(logger, entity);
        if (pdfBytes != null) {
            builder.setContentType(ContentTypes.PDF);
            builder.setStatusCode(200).setBinaryResponse(pdfBytes);
        } else {
            ErrorResponse response = new ErrorResponse("Could not create record sheet for " + path);
            builder.setStatusCode(500).setJsonResponse(response);
        }
    }

[neoancient]

When I try to print a test 200-page document I get to about 14% (very slowly) and it runs out of memory and barfs.

[neoancient]

The sheet instances take up a lot more memory when processed, and large jobs can easily run out of memory. I tried clearing generated data from the PrintRecordSheet instance without success. The only way I found to handle the memory problems is to remove the sheets once they're added to the document. I don't see any reason the book needs to be reusable.

[sixlettervariables]

I'll test this with a large document tonight, good to know that was the issue.

I found some other items that kept large amounts of memory rooted and will put up a separate PR.