Expose structured stats to application

[rosenhouse]

Feature request: An application may need to monitor splinterdb stats and do its own processing on that data (e.g. push data to an external metrics aggregator, expose data in a UI, etc).

Possible solution: We could add a simple API in the splinterdb.h layer that allows the application to retrieve stats, for example something roughly like

typedef struct {
   ... // contains numeric fields for the various stats
       // that get printed by the current stats-printing code
} splinterdb_stats;

void splinterdb_stats_get(splinterdb *kvs, splinterdb_stats *stats_out);