http: refuse to deal with >4GB chunks in uploads

http: refuse to deal with >4GB chunks in uploads

The `hex' perlop will return an NV (typically 64-bit double) on
UV (unsigned int) overflow and warns on larger values.  While
64-bit integer builds of 32-bit perl (e.g. Debian i386) can
handle 64-bit numbers, there are builds of perl which still use
32-bit integers nowadays (e.g. OpenBSD 7.x i386).

It's unlikely we'll ever see chunks even close to 4GB, so just
cap it at 8 hex characters and drop clients which send larger
amounts.