fix: reduce memory usage in group chats by 75%

Significantly reduced the memory usage of groups since all message slots
are preallocated for every peer for send and receive buffers of buffer
size (hundreds of MiB peak when save contained alot of peers to try to
connect to)

toxcore/group_common.h

@@ -52,7 +52,7 @@
 #define MAX_GC_PACKET_SIZE (MAX_GC_PACKET_CHUNK_SIZE * 100)
 
 /* Max number of messages to store in the send/recv arrays */
-#define GCC_BUFFER_SIZE 8192
+#define GCC_BUFFER_SIZE 2048
 
 /** Self UDP status. Must correspond to return values from `ipport_self_copy()`. */
 typedef enum Self_UDP_Status {