http: reduce parser from 72 to 56 bytes on 64-bit

http: reduce parser from 72 to 56 bytes on 64-bit

This allows the parser struct to fit in one cache line on
x86-64 systems where cache lines are 64 bytes.

Using 32-bit integer lengths is safe here because these are only for
tracking offsets within the HTTP header buffer.  We can safely limit
HTTP headers and in-memory buffers to be less than 4GB without
anybody complaining.

HTTP bodies continue to use off_t (usually 64-bit, even on 32-bit
systems) sizes and support as much as the OS/hardware can handle.