asyncd: fix races in writing to spool

Add a simple lock to the file. It avoids mixing data that will
disrupt the parsing when reading it back by munin-update.
asyncd: typo in pod
asyncd: try to reuse socket when asking node

We use one socket per process per run. So if --fork is
active, there will be no socket reuse.
asyncd: add the hostname in $0

This ease the fact when multiple async run on the same host
asyncd: fix timeout bug & adding some debug
asyncd: ignore current plugin if failing cnx to node

We should not die when failing to connect to a node. Just move
forward to the next plugin to fetch.
asyncd: close the datafile before rename

The flush might only be done on close, which was done on scope end
: ie *after* rename.

As the flush might only be done on close, external readers migh
then only see partial data, which defeats the whole purpose of the
atomic rename().

This commit fixes this race condition.
asyncd: quickfix a sleeping dataloss bug

When sleeping too little, the code just discards every plugin, and
reverts to sleep $timeout sec.

This is a dataloss bug, so it has to be fixed hastily.  An other
commit will fix it better.

Thx to trippeh on IRC for showing me the bug *very* clearly.