node/asyncd: fix a crash case

The asyncd would die() upon failure to open a new socket to the node when
--fork was not enabled.

This commit just emits a warning, and simply try again for the next plugin.
That way only a plugin fetch will fail, not the whole asyncd process.
node/asynd: avoid wake up each second
master: fix unexpected "warning limit" lines

Quoted from the ticket:

    Data sources specified as a negative value of another field default to a
    warning limit of 2. This results in an unexpected horizontal red line on
    some graphs. Cause is a small error originating from r3143

Thanks tpalfalv for the report and the patch !

Closes #910