Integrate prefetch functionality into msui client to speed up map loading

[Marc R.]

Idea: While the user views the current map, do a background-prefetch of those maps that the user is most likely to view next.

• probably requires one "prefetch thread" per WMS control that controls a "prefetch queue"
• if the user issues a new WMS request, all prefetch-tasks need to be removed from the "prefetch queue"; new tasks could be added according to a prefetch_config, e.g.:

prefetch_config =
    { ["validtime_fwd", 3],
       ["validtime_bwd", 3],
       ["lvl_up", 2],
       ["lvl_up", 2],
       ["inittime_fwd", 2] } 

[Reimar Bauer]

look at old devel branch

[Reimar Bauer]

Initial draft for prefetching images from WMS Server.

This, so far, cannot prefetch levels and does not cancel the queue of queued prefetch calls. There may also be ugly race conditions, if the pre-fetched file is not ready yet, as the main thread will require another Map. That is why it is disabled by default.

See Issue #2

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