httputil: Fix quadratic performance of repeated header lines

httputil: Fix quadratic performance of repeated header lines

Previouisly, when many header lines with the same name were found
in an HTTP request or response, repeated string concatenation would
result in quadratic performance. This change does the concatenation
lazily (with a cache) so that repeated headers can be processed
efficiently.

Security: The previous behavior allowed a denial of service attack
via a maliciously crafted HTTP message, but only if the
max_header_size was increased from its default of 64kB.