Expose structured stats to application

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);

Start in splinterdb.h and locate the current stats-printing code to identify the numeric fields exposed there. Define the structured stats API around those values, then verify that an application can retrieve the same statistics for its own processing.