metrics: queue partial response writes

write_all() previously did a blocking-style loop on a O_NONBLOCK
socket and bailed on the first EAGAIN. With a slow scrape client or
a small kernel send buffer that meant the response was truncated and
the connection dropped mid-flight.

Allocate one contiguous xmit_buf per response holding header + body,
then drain it in xmit_flush():

  * full write → mark the client for disconnect on the next event
    loop tick;
  * EAGAIN/EWOULDBLOCK → enable MD_MODE_WRITE so cln_write() resumes
    the drain when the socket becomes writable;
  * hard error → mark for disconnect, caller tears down.

cln_read() now stops reading once a response is queued (read events
during the response phase are uninteresting since we'll close on
flush), and cln_write() finishes the drain and disconnects when the
last byte is out. The existing per-client read timer doubles as a
write deadline, so a peer that opens the connection and never reads
still gets cleaned up after read_timeout seconds.

Smoke-tested with a python client that uses SO_RCVBUF=256 and
sleep(0.05) between recv()s — it now reads the entire ~2.8 KiB body
across many short reads. Five concurrent slow readers plus a fast
scrape all complete successfully.

Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>

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