Improve performance and user experience for long running tasks

[knabar]

Saving rendering settings to large number of images can take a very long time.

Two PRs are open now to start addressing this issue:

• Block UI for some long running tasks #543
• Remove webgateway cache #549 to remove non-performant cache invalidation

Further work will attempt to reduce the time these tasks take, and to block UI interaction for other long-running tasks during which the user should not interact further.

• Batch size changed to 1 in #5192, investigate why
• Investigate other speed improvements for "Save to all"/"Paste"
• Investigate adding a way for clients to find out how busy the server is, to tell user or change behavior