Pause and resume import #229

WordPress importer struggles with large datasets. Once it crashes, there's no easy way to resume. Let's change that.

High-level steps to get there:

• Separate importing logic for every supported entity type #228
• Streaming ID and author mapping (without accumulating global state or running table-wide UPDATE queries)
• Save importer cursor, resume from the last stored cursor

Related to WordPress/php-toolkit#138