18941: Separate get() behavior for prefetch

18941: Separate get() behavior for prefetch

Add flag to get() which causes it to return None immediately if a cache
slot already exists for a block.

In the standard behavior, if multiple readers try to get() the same
block, the first one will start downloading the block, and all the
others will wait and return the block content from the cache slot
when complete.

With the new optional behavior, if multiple readers try to get() the same
block, the first one will start downloading the block, and all the
others will immediately return None.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>