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ARROW-7004: [Plasma] Make it possible to bump up object in LRU cache

To avoid evicting objects too early, we sometimes want to bump up a number of objects up in the LRU cache. While it would be possible to call Get() on these objects, this can be undesirable, since it is blocking on the objects if they are not available and will make it necessary to call Release() on them.

Closes #5741 from pcmoritz/plasma-lru-refresh and squashes the following commits:

5fd639274 <Philipp Moritz> fix test
486e8a043 <Philipp Moritz> update
f68223d51 <Philipp Moritz> update
0b3e84bad <Philipp Moritz> comments
ae831a593 <Philipp Moritz> docs
9b9c0615b <Philipp Moritz> add test
2f62bf705 <Philipp Moritz> Implement LRU refreshing for objects, first take

Authored-by: Philipp Moritz <pcmoritz@gmail.com>
Signed-off-by: Philipp Moritz <pcmoritz@gmail.com>