repair: convenience function for syncing a range
repair: convenience function for syncing a range

This patch adds a function sync_range() for synchronizing all partitions
in a given token range between a set of replicas (this node and a list of
neighbors).
Repair will call this function once it has decided that the data the
replicas hold in this range is not identical.

The implementation streams all the data in the given range, from each of
the neighbors to this node - so now this node contains the most up-to-date
data. It then streams the resulting data back to all the neighbors.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

duarten/scylla,scylladb/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,duarten/scylla