Adding real allocator fragmentation to INFO and MEMORY command + active defrag test

other fixes / improvements:
- LUA script memory isn't taken from zmalloc (taken from libc malloc)
  so it can cause high fragmentation ratio to be displayed (which is false)
- there was a problem with "fragmentation" info being calculated from
  RSS and used_memory sampled at different times (now sampling them together)

other details:
- adding a few more allocator info fields to INFO and MEMORY commands
- improve defrag test to measure defrag latency of big keys
- increasing the accuracy of the defrag test (by looking at real grag info)
  this way we can use an even lower threshold and still avoid false positives
- keep the old (total) "fragmentation" field unchanged, but add new ones for spcific things
- add these the MEMORY DOCTOR command
- deduct LUA memory from the rss in case of non jemalloc allocator (one for which we don't "allocator active/used")
- reduce sampling rate of the rss and allocator info

src/zmalloc.h

@@ -79,8 +79,8 @@ void zfree(void *ptr);
 char *zstrdup(const char *s);
 size_t zmalloc_used_memory(void);
 void zmalloc_set_oom_handler(void (*oom_handler)(size_t));
-float zmalloc_get_fragmentation_ratio(size_t rss);
 size_t zmalloc_get_rss(void);
+int zmalloc_get_allocator_info(size_t *allocated, size_t *active, size_t *resident);
 size_t zmalloc_get_private_dirty(long pid);
 size_t zmalloc_get_smap_bytes_by_field(char *field, long pid);
 size_t zmalloc_get_memory_size(void);