SwingStore hostStorage setExportCallback is unsafe

[gibson042]

Describe the bug

As documented, "hostStorage.setExportCallback() is used to register an export callback after swingstore creation [but] Most applications will instead provide options.exportCallback to openSwingStore()". But looking at the implementation, noteExport performs a database write if and only if there is a defined exportCallback at the time. This means that calling setExportCallback too late will result in the callback seeing only a subset of exports when it is later called, which is a footgun.

Proposed solution

We should just remove the method, since the exportCallback option provides equivalent functionality and doesn't suffer from the same issue (and in fact we never even use setExportCallback, even in testing).

[mhofman]

Possibly related: #9655