Identity document changed

Original changes:

• https://github.com/ucoin-io/ucoin/issues/266 (blocktime reference instead of arbitrary time)
• https://github.com/ucoin-io/ucoin/issues/262 (SHA1 -> SHA256)
• https://github.com/ucoin-io/ucoin/issues/327 (Version: 2)

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There is now an Identity document, previously known as "self-certification". It has a new structure like the following example taken from protocol:

Version: 2
Type: Identity
Currency: beta_brousouf
Issuer: HgTTJLAQ5sqfknMq7yLPZbehtuLSsKj9CxWN7k8QvYJd
UniqueID: lolcat
Timestamp: 32-DB30D958EE5CB75186972286ED3F4686B8A1C2CD
J3G9oM5AKYZNLAB5Wx499w61NuUoS57JVccTShUbGpCMjCqj9yXXqNq7dyZpDWA6BxipsiaMZhujMeBfCznzyci

The main changes are:

• A full document with fields, notably headers Version, Type and Issuer
• Field Version now equals 2
• Timestamp is now a blocktime reference (or BLOCK_UID), so an identity cannot be created with an arbitrary future time