from Botond.
Recently it was observed (#6603) that since 4e6400293ea, the staging
reader is reading from a lot of sstables (200+). This consumes a lot of
memory, and after this reaches a certain threshold -- the entire memory
amount of the streaming reader concurrency semaphore -- it can cause a
deadlock within the view update generation. To reduce this memory usage,
we exploit the fact that the staging sstables are usually disjoint, and
use the partitioned sstable set to create the staging reader. This
should ensure that only the minimum number of sstable readers will be
opened at any time.
Refs: #6603Fixes: #6707
Tests: unit(dev)
* 'view-update-generator-use-partitioned-set/v1' of https://github.com/denesb/scylla:
db/view: view_update_generator: use partitioned sstable set
sstables: make_partitioned_sstable_set(): return an sstable_set
e4b74356bb