Feature/combined writes - one packet can ship multiple writes

[jthomas43]

No description provided.

[jthomas43]

currently this patch allocates stream packets that look like this:

packet_t {...};
uv_buf_t header;
uv_buf_t padding;  // placeholder for mtu probe padding. zero length unless mtu probe sent
uv_buf_t data[n];  // payload pointers for n writes
udx_write_t*[];    // udx_write_t pointers for n writes

I think I should do another pass to remove the always-allocated padding field. it will complicate the code but 1. may be faster and 2. there may be a bug on some android when sendmsg is given an empty buffer.