"ipfs add" calculate CID and check for it *before* uploading to API server.

ipfs add should calculate the CID and check for it's existence on the api server / network *before* copying the data over.

An example scenario:

* I rsync 67GiB worth of files to my system
* I ```ipfs --api=$IPFS_API_SERVER add -r /cache/*``` to make them available.

In the above scenario, my remote ipfs client seemingly needs to upload 67.88GiB of files to the API server every time I sync to IPFS. Ideally the local ipfs binary could quickly generate a CID and check for the CID's existance *before* transferring the data to the remote API server. Even in local deployments this could save time since IPFS unnecessarily transferring files.