HTTPDecoder: restore old limit of 80 kB headers

[weissi]

Motivation:

NIO's historic limit was that the maximum header block was limited to 80 kB which is a sensible limit and originates from Nginx's http_parser.c.

The history of the changes:

• commit e67fc99: Introduces the 80 kB limit
• commit 6918034: Accidentally removes the limit entirely
• commit b24872d: Introduced the 2 MB / 256 headers limit

I would argue that 2MB & 256 headers is a bad limit:

• 2MB is way too big, that allows pretty powerful slowloris-style attacks forcing NIO to hold on to 2 MB per connection
• 256 headers is too little for software that may be a proxy
  • in HTTP, the number of headers is non-semantic
    • A: a: 1\r\n a: 2\r\n a: 3\r\n a: 4\r\n a: 5\r\n a: 6\r\n[...]\r\na: 257\r\n
    • B: a: 1, 2, 3, 4, 5, ..., 257\r\n
    • A & B are equivalent. Yet, A would be rejected and B accepted. That's not right.

Modifications:

Restore the old limits.

Result:

• More compatible (because we allow more than 256 headers)
• More secure (because the default goes back to 80kB)