Rsp: optimize in-memory size

The in-memory size of Rsp can be made smaller. Currently, it looks like this:

public class Rsp {
    protected boolean         received;
    protected boolean         suspected;
    protected boolean         unreachable;
    protected final Address sender;
    protected T                    retval;
    protected Throwable     exception;
}

Optimizations:

1. received, suspected and unreachable can be compacted into a byte field (flags)
2. retval and exception can be merged into Object value
3. sender can be removed: RspList already has the sender

New format:

public class Rsp {
    protected byte             flags;
    protected Object           value;
}

Size of Rsp is 32 bytes (used JOL to measure) before, and 24 bytes after the changes.