When starting repair, we divided the large token ranges (vnodes) linto small
subranges of a desired length (around 100 partition), and built a huge list
of those subranges - to iterate over them later and compare checksums of
those chunks.
However, building this list up-front is completely unnecessary, and wastes
a lot of memory: In a test with 1 TB of data, as much as 3 gigabytes was
spent on this list. Instead, what we do in this patch is to find the next
chunk in a DFS-like splitting algorithm, using only the token range
midpoint() function (as before). The amount of memory needed for this is
O(logN), instead of O(N) in the previous implementation.
Refs #2430.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
