It might take long time for get_all_ranges_with_sources_for and
get_all_ranges_with_strict_sources_for to calculate which cause reactor
stall. To fix, run them in a thread and yield. Those functions are used in
the slow path, it is ok to yield more than needed.
Fixes #3639
Message-Id: <63aa7794906ac020c9d9b2984e1351a8298a249b.1536135617.git.asias@scylladb.com>
(cherry picked from commit 8edf3defdf)