Introduce accountId field which is a new id (SHA-1 hash) for an Account.

There are no getter/setters defined since they are referencing
oldAccountId at the moment. Once all references of old ids is removed from
Gerrit getters and setters will point at this field.

Change-Id: I8f97a467f5877e629344eea3ff6f636eb1f6caf3
Signed-off-by: Grzegorz Kossakowski <grek@google.com>