Nyzo 639: nickname transactions

Nyzo version 639 (commit on GitHub) modifies the nickname metadata transactions to work with the NicknameScript.

This version affects the verifier, client, and sentinel.

The NicknameScript was added to version 638. This script produces a nickname index from the transactions sent to id__8ezA79npaBzn430Y1fset6L5bZHcXhuVsYc0_1mEd8uatj3-NAMe. Version 567 added on-chain metadata through small transactions sent automatically by verifiers. This version modifies the nickname metadata functionality of version 567 to be compatible with the script of version 638.

In MetadataManager, the nicknameMetadataKey that was used for the previous metadata transaction format was replaced with the identifier of the account where the NicknameScript is registered.

In MetadataManager.registerBlock(), logic recognizing the old nickname transaction format was replaced with logic recognizing the new nickname transaction format.

The MetadataManager.nicknameTransaction() method now produces the nickname transaction in the new format.

The MetadataManager.processNicknameTransaction() method now uses the new format, retrieving the nickname from the transaction's sender data.

The localNickname field and its supporting methods were removed from NicknameManager. This field was redundant with the nickname provided by the Verifier.getNickname() method.

In the Verifier class, some inconsequential cleanup of the code was done.

Later in the Verifier class, more inconsequential cleanup was done, a call to NicknameManager.setLocalNickname() was removed because that method was removed, and the getDefaultNickname() method was added to assist in detection of whether a user actually set a nickname or if the nickname based on the raw verifier identifier is in use.