Added an unique ID field to every slow log entry.

src/slowlog.h

@@ -2,6 +2,7 @@
 typedef struct slowlogEntry {
     robj **argv;
     int argc;
+    long long id;       /* Unique entry identifier. */
     long long duration; /* Time spent by the query, in nanoseconds. */
     time_t time;        /* Unix time at which the query was executed. */
 } slowlogEntry;