Adding, removing and updating keys with ADD_KEY, REM_KEY, MOD_KEY

[lovesh]

Currently, to add a new key, the spec specifies that the whole DID doc with the new key added must be submitted to the ledger. Similarly for removing and updating keys, the whole DID doc has to be submitted.
The proposal is to have separate transactions for each action so the whole DID doc does not need to be sent and it is efficient for the ledger to verify compared to calculating a delta between the submitted doc and existing doc.

1. Adding a new key is done with ADD_KEY txn.
2. Removing a new key is done with REM_KEY txn.
3. Updating an existing key is done with MOD_KEY txn