Expand account storage mode options

[bobbinth]

Currently, account storage options are: (1) on chain and (2) off chain. These options a encoded in the 3rd most significant bit of the account ID. It may be good to use 2 bits rather than 1 to encode this information as we may want to add additional storage modes in the future. Specifically, we could do something like this:

• 00: account data is stored on-chain in plaintext.
• 01: account data is stored on-chain in encrypted text.
• 10: reserved for future use.
• 11: account data is stored off chain.

Thus, we'd have the first 4 bits of the account ID encode account properties: 2 bits for account type and 2 bits for account storage mode.

[bobbinth]

Superseded by #140.