Reorder fields to reduce `sizeof(asio_event_t)` to 40 bytes

[mneumann]

Reduces sizeof(struct asio_event_t) from 48 bytes to 40 bytes on 64-bit x86 Linux/FreeBSD. The noisy field was talking up 3 bytes due to aligment of the following 64-bit value nsecs at a word boundary (offset 32). The two other flags readable and writable fit in the alignment hole and no longer waste 8 bytes to align the whole struct at word boundaries again.

Note that the offset of nsecs is not changed. It still lives at offset 32.