mdbx: add pgop_stat.gcrtime for collect the time spent loading and searching inside GC.

mdbx.h

@@ -2491,6 +2491,9 @@ struct MDBX_envinfo {
     uint64_t unspill; /**< Quantity of unspilled/reloaded pages */
     uint64_t wops;    /**< Number of explicit write operations (not a pages)
                            to a disk */
+    uint64_t
+        gcrtime_seconds16dot16; /**< Time spent loading and searching inside
+                                     GC (aka FreeDB) in 1/65536 of second. */
   } mi_pgop_stat;
 };
 #ifndef __cplusplus