headers: enforce a max number of response header to accept

The limit is 5000 headers in a single transfer. To avoid problems caused
by mistakes or malice.

Add test 747 to verify

Reported-by: wolfsage on hackerone

Closes #17281

tests/data/test498

@@ -20,7 +20,7 @@ Content-Length: 6
 Location: /
 Connection: close
 Content-Type: text/html
-%repeat[1700 x Repeat-this-Header-a-large-number-of-times: Dorothy lived in the midst of the great Kansas prairies, with Uncle Henry, who was a farmer, and Aunt Em, who was the farmer’s wife.%0a]%
+%repeat[1700 x Repeat-this-Header-a-large-number-of-times: Dorothy lived in the midst of the great Kansas prairies, with Uncle Henry, who was a farmer, and Aunt Em, who was the farmer's wife. Their house was small, for the lumber to build it had to be carried by wagon many miles. There were four walls, a floor and a roof, which made one room; and this room contained a rusty looking cookstove, a cupboard for the dishes, a table, three or four chairs, and the beds. Uncle Henry and Aunt Em had a big bed in one corner, and Dorothy a little bed in another corner. There was no garret at all, and no cellar--except a small hole dug in the ground, called a cyclone cellar, where the family could go in case one of those great whirlwinds arose, mighty enough to crush any building in its path. It was reached by a trap door in the middle of the floor, from which a ladder led down into the small, dark hole.%0a]%
 -foo-
 </data>
 </reply>