call: separate function invocation from result encoding

The iproto subsystem switches between two output buffers once in a while
in order to reclaim memory so passing a pointer to the output buffer
directly to box_process_call() or box_process_eval() is incorrect in
case the called function yields. To fix that, let's make these functions
return the CALL/EVAL result in a port object, which then can then be
encoded in msgpack with port_dump().

Needed for #946