mthread: reduce memory usage

mthread keeps header data of all messages in memory until it exits without
ever using it. With this change it frees the header data of each message
right after processing it.

Closes: #222 [via git-merge-pr]

mthread.c

@@ -29,7 +29,6 @@ static int optional;
 struct container {
 	char *mid;
 	char *file;
-	struct message *msg;
 	time_t date;
 	struct container *parent;
 	struct container *child;
@@ -87,7 +86,6 @@ midcont(char *mid)
 			exit(111);
 		c->mid = mid;
 		c->file = 0;
-		c->msg = 0;
 		c->date = -1;
 		c->optional = 0;
 		c->parent = c->child = c->next = 0;
@@ -104,7 +102,6 @@ store_id(char *file, struct message *msg)
 
 	c = midcont(mid(msg));
 	c->file = strdup(file);
-	c->msg = msg;
 	c->optional = optional;
 
 	return c;
@@ -239,6 +236,8 @@ out2:
 			c->child = 0;
 		}
 	}
+
+	blaze822_free(msg);
 }
 
 time_t
@@ -290,7 +289,6 @@ find_roots()
 	top = malloc(sizeof (struct container));
 	if (!top)
 		exit(111);
-	top->msg = 0;
 	top->date = -1;
 	top->file = 0;
 	top->next = top->child = top->parent = 0;
@@ -315,7 +313,6 @@ prune_tree(struct container *c, int depth)
 			// turn into child if we don't exist and only have a child
 			c->mid = c->child->mid;
 			c->file = c->child->file;
-			c->msg = c->child->msg;
 			if (c->child->date > c->date)
 				c->date = c->child->date;
 			c->optional = c->child->optional;