This patch adds a new type of message, "REPAIR_CHECKSUM_RANGE" to scylla's
"messaging_service" RPC mechanism, for the use of repair:
With this message the repair's master host tells a slave host to calculate
the checksum of a column-family's partitions in a given token range, and
return that checksum.
The implementation of this message uses the checksum_range() function
defined in the previous patch.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>