node: Only spoolfetch send 5 samples per service

This enables to not overwhelm the munin-update when there is big backlog
to handle.  Note that this is only a 5x speed increase, so a backlog of
5 hours is fully sent in 1 hour.

This value is _very_ conservative, but a new config value is planned in
the devel branch. Here in stable-2.0, it's a perfectly valid strategy.
asyncd: Use SyncDictFile for plugin_rates

That way we have a text-only DB, which is fully-multiprocess aware.
asyncd: just use the SpoolWriter META-DATA

This avoids completely another new file.
Merge branch 'fix-spool' into stable-2.0

Conflicts:
	node/lib/Munin/Node/SpoolReader.pm
This corrects the truncating of negative values in the diskstats plugin
and calculates its value based on the evidence that this is a overflow
value.

This patch was created by ksmadsen in
http://munin-monitoring.org/ticket/1275

The description provided was:

I've traced this problem to the fact that on the RH kernel we are using,
the rd_ticks/wr_ticks in /sys/block/*/stat are 32-bit numbers, so when
the ticks wraps, a negative value is reported.

Looking at the kernel source code, the tick counters are internally all
64-bit, but when they are read from the stat file, the output is limited
to 32-bits.
(cherry picked from commit 91a9e62b14957cabcd8fcfceeb6f656b95eb2261)