Add cluster/multiprocess support

[pkarw]

We need to modify the attribute.mapToVS to work on global/shared attributes hash

Attribute values are populated by each process/product import - so if the state weren't shared it would have been easily desynchronized. Right now because we don't support multiprocess it's pretty straightforward: using a local hash.

Here is the function to be modified with some notes on the possible solutions:
\

function mapToVS (attributeCode, attributeType, attributeValue) {
    let attr = attrHash[attributeCode]
    if (! attr) {
        attr = attributeTemplate(attributeCode, attributeType)
        attr.id = maxAttrId
        attr.attribute_id = maxAttrId
        
        attrHash[attributeCode] = attr
        maxAttrId++
    } 
    if (attr.frontend_input == 'select') {
        let existingOption = attr.options.find((option) => { return option.label == attributeValue})
        if(!existingOption) {
            let lastOption = attr.options.length > 0 ? attr.options[attr.options.length-1] : null // we can use memored or elastic search to store each option per each attribute separately - to keep the same indexes between processes for example key would be: $attribute_code$$attribute_value = 14 
                                                                                                  // OR SEND MODIFIED attributes to the workers each time attrHash changes: https://nodejs.org/api/cluster.html#cluster_cluster_workers
            let optIndex = 1
            if (lastOption) {
                optIndex = lastOption.value + 1
            }
            attr.options.push({
                label: attributeValue,
                value: optIndex
            })
            return optIndex
        } else {
            return existingOption.value // non select attrs
        }


    } else {
        return attributeValue
        // we're fine here for decimal and varchar attributes
    }
}