incremental transcription export sync needs to be single threaded (can't run in signal handler)

[rlskoeser]

this is a new approach to functionality you already tested on #1096

Please test again the same way:

• edit an existing transcription and make a few text changes
• edit an existing transcription that has content on more than one image; remove blocks of content so there is only content on image (one of the annotation list files should be removed;
• add a new transcription
• remove an existing transcription entirely — the corresponding exports should be removed from github

updates are being pushed to this test branch: https://github.com/Princeton-CDH/test-pgp-annotations/tree/test-tei-migration2

[rlskoeser]

Thanks @kseniaryzhova for testing all the cases listed in the issue notes!

Ben and I just tested making simultaneous edits on different documents, and confirmed that the export script created two different commits, with each of us co-author on the change for the record we modified.