eve/http: add request/response http headers

eve/http: add request/response http headers

Add a keyword configuration dump-all-headers, with allowed values
{both, request, response}, dumping all HTTP headers in the eve-log http
object. Each header is a single object in the list request_headers
(response_headers) with the following notation:

{
    "name": <header name>,
    "value": <header value>
}

To avoid forged malicious headers, the header name size is capped at 256
bytes, the header value size at 2048.

By default, dump-all-headers is disabled.