Fix PDF preview and rendition of Mail document

HTML preview (based on the PDF rendition) and PDF rendition of Mail documents always produce the same PDF, the first one that got generated since the Nuxeo Platform startup.

How to reproduce:

1. install addon nuxeo-imap-connector and start a Nuxeo instance
2. create a MailFolder linked to a mail account
3. navigate to one if the imported Mail document, the HTML preview produces the proper preview
4. navigate to another imported Mail document

Expected result: The HTML preview produces the proper preview and the PDF rendition is correct.
Actual result: The HTML preview produces the preview of the first previewed Mail document and the PDF rendition returns the PDF of the first previewed mail document.

This is due to the fact that the digest generated in the Mail document type's blob holder is hardcoded: https://github.com/nuxeo/nuxeo/blob/10.10/nuxeo-features/nuxeo-platform-mail/nuxeo-platform-mail-core/src/main/java/org/nuxeo/ecm/platform/mail/adapter/MailMessageBlobHolder.java#L67.