Address collision results in public keys being overwritten

[karmacoma]

In the event of an collision where the same passphrase resolves to the same LSK address, the public key on mem_accounts is overwritten, allowing transactions to be signed from the account using either of the colliding passphrases.

Initial resolution:

• Change the behavior of the endpoints POST /api/accounts/open and POST /api/accounts/generatePublicKey, so that new mem_accounts entries are not created for unregistered addresses.
• Change the behavior of the endpoint: PUT /api/transactions and other endpoints for various transaction types, so that a new mem_accounts entry is created, if the account is unregistered.
• Make the publicKeyfor each entry in mem_accounts immutable.

Further mitigations:

• Increase address size to reduce probability of collisions.
• Deprecate the use of addresses and use only pubic keys.