Reduce memory used by `ndpiReader` (#1371)

`ndpiReader` is only an example, aiming to show nDPI capabilities
and integration, without any claim about performances.

Nonetheless its memory usage per flow is *huge*, limiting the kinds
of traces that we can test on a "normal" hardware (example: scan
attacks).

The key reason of that behaviour is that we preallocate all the memory
needed for *all* the available features.

Try to reduce memory usage simply allocating some structures only
when they are really needed. Most significant example: JOY algorithms.

This way we should use a lot less memory in the two most common
user-cases:
 * `ndpiReader` invoked without any particular flag (i.e `ndpiReader -i
$FILENAME_OR_IFACE`)
 * internal unit tests

Before (on x86_64):
```
struct ndpi_flow_info {
[...]
	/* size: 7320, cachelines: 115, members: 72 */
```
After:
```
struct ndpi_flow_info {
[...]
	/* size: 2128, cachelines: 34, members: 75 */
```

1 files changed, 6 insertions, 6 deletions

diff --git a/example/reader_util.h b/example/reader_util.h
index 0c584b378..01955129a 100644
--- a/example/reader_util.h
+++ b/example/reader_util.h
@@ -134,7 +134,6 @@ struct flow_metrics {
 
 struct ndpi_entropy {
   // Entropy fields
-  pkt_timeval src2dst_last_pkt_time, dst2src_last_pkt_time, flow_last_pkt_time;
   u_int16_t src2dst_pkt_len[MAX_NUM_PKTS];                     /*!< array of packet appdata lengths */
   pkt_timeval src2dst_pkt_time[MAX_NUM_PKTS];               /*!< array of arrival times          */
   u_int16_t dst2src_pkt_len[MAX_NUM_PKTS];                     /*!< array of packet appdata lengths */
@@ -192,14 +191,15 @@ typedef struct ndpi_flow_info {
   ndpi_protocol detected_protocol;
 
   // Flow data analysis
+  pkt_timeval src2dst_last_pkt_time, dst2src_last_pkt_time, flow_last_pkt_time;
   struct ndpi_analyze_struct *iat_c_to_s, *iat_s_to_c, *iat_flow,
     *pktlen_c_to_s, *pktlen_s_to_c;
 
   char info[255];
   char flow_extra_info[16];
   char host_server_name[240];
-  char bittorent_hash[41];
-  char dhcp_fingerprint[48];
+  char *bittorent_hash;
+  char *dhcp_fingerprint;
   ndpi_risk risk;
   
   struct {
@@ -229,13 +229,13 @@ typedef struct ndpi_flow_info {
   } http;
 
   struct {
-    char username[32], password[32];
+    char *username, *password;
   } telnet;
 
   void *src_id, *dst_id;
 
-  struct ndpi_entropy entropy;
-  struct ndpi_entropy last_entropy;
+  struct ndpi_entropy *entropy;
+  struct ndpi_entropy *last_entropy;
 
   /* Payload lenght bins */
 #ifdef DIRECTION_BINS