Save dirty attachments contents when saving the document

The goal is to:

• save attachment contents marked dirty during XWiki(Hibernate|Jcr)Store#saveXWikiDoc()
• optimize XWiki(Hibernate|Jcr)AttachmentStore#saveAttachmentContent() to only save contents when dirty
• remove call to XWikiDocument#saveAllAttachements() that becames uneeded

See this thread for details and vote: http://markmail.org/thread/4kfxallt3dyorezr