In origin, there are two APIs to get the information about the current
running compactions. Both APIs do the string formatting.
This patch changes the API to have a single API get_compaction that
would return a list of summary object.
The jmx would do the string formatting for the two APIs.
This change gives a better API experience is it's better documented and
would make it easier to support future format changes in origin.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>