Attachment store optimisations

[andris9]

Currently attachments stored in GridStore can not be shared between different messages because these contain message specific metadata. The only metadata that should remain should be:

• messages
• transferEncoding

When storing new attachments the server should try to find an existing attachment first

database.collection('attachments.files').findOneAndUpdate({
        md5: attachmentMd5,
        length: attachmentLength
    }, {
        $addToSet: {
            messages: messageId
        }
    }, (err, res)=>{
        // if a document was updated (res.value exists) then thats it
        // otherwise create a new attachment instance
    });

This would assume a compound index on md5+length

[andris9]

Something that needs be solved first are too large indexed arrays. If there's a lot of messages that include the same attachment then the messages array might grow very large

[andris9]

Removed extra fields from attachment metadata that are not needed with 8c7cedc

[andris9]

Instead of messages array store referenced message count as the attachment id is stored in mail document anyway

[andris9]

Attachments are deduplicated from v1.0.33. Deduplication key is a sha256 hash field. Existing md5 is not used.