The schedule_repair() operates on a map of endpoint:mutations pairs.
Next patch will need to extend this entry and it's going to be easier if
the entry is wrapped in a helper structure in advance.
This is where the forwardable reference cursor from the previous patch
gets its user. The schedule_repair() produces a range of rvalue
wrappers, but the create_write_response_handler accepting it is OK, it
copies mutations anyway.
The printing operator is added to facilitate mutations logging from
mutate_internal() method.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>