Move JsonIterator to NMT

[iuravic]

Refactor from here to https://github.com/Automattic/newspack-migration-tools/pull/19 Also moved the following:

• src/Utils/BatchLogic.php, because it is used by JsonInterator
• src/Utils/Logger.php, because JsonInterator uses it

[iuravic]

Because the namespace changed in the NMT, we'd need to run a search and replace on NewspackCustomContentMigrator\Utils\BatchLogic -> Newspack\MigrationTools\Util.

We need to keep the logger class here. See the NMT PR

Ah, of course, how clumsy of me. Thank you @naxoc, now fixed in 221debb.

~Also in 6624e97 now updated usage of Logger from NewspackCustomContentMigrator\Utils\Logger to Newspack\MigrationTools\Util\Logger.~ I now saw your note in the other PR about the Logger. Ok, I'll get acquainted with those other two logger classes in the NMT, and un-move the Logger itself.

JsonIterator itself is only used by one archived migration, so no need to change any of that.

[iuravic]

@naxoc , ready for re-check,

• brought back the Logger here 754b043
• fixed new namespace for BatchLogic here 221debb