fix(bulk): Load IDs from list only when necessary

[kwahlin]

When generating changeSets we are only interested in the diff between matchForm and targetForm and thus we don't need to bother with loading ID lists possibly contained in matchForm. Loading ID lists over and over for every call to the bulk preview API is likely the reason for the heap space errors occurring lately.

The key fix is to not provide a Whelk instance here

librisxl/housekeeping/src/main/groovy/whelk/housekeeping/BulkChangePreviewAPI.java

Line 97 in e91e02a

result.put("changeSets", update.getTransform().getChangeSets());

while the rest is to clarify a bit what's going on when loading IDs.

[jannistsiroyannis]

LGTM!