metadatasvc: changed sha1 to sha256

Although HMAC with sha1 is less affected by collisions than sha1 alone I personally think it's better to use sha256 since the beginning.
This has also the advantage of automatically increasing the key sizei, for the HMAC function, from 160 bits to 256, which consequently increases the HMAC strength.
Merge pull request #135 from silenteh/metadatasvc-sha256

metadatasvc: changed sha1 to sha256