Use static variant to allow the types of vesting balances to be easily
extended and the creation operation allows for many different types of
initialization parameters.
Added a check that requires a minimum claim date which allows creating
of vesting balance objects with a cliff.
Reasons:
1. The protocol should not depend upon implementation details such as
how the database objects are structured or reflected
2. The protocol should deal in abstract concepts
3. Should use fc::datastream rather than istringstream for performance
and memory allocation reasons
4. Fees should be charged proportional to the size of the operation
5. Validate on the assert operation should also perform sanity checks
on types
6. Protocol definition objects should never depend upon the database
because they may be used in situations where the database and
evaluators are not present.
7. Reflected field names should never have '_' in them because they
become part of the *PUBLIC* json definition.
Declare, but not yet implement, account_upgrade_operation which can
upgrade accounts to members.
Resolve#21 by removing all the sketchy pointer arithmetic stuff
