proxmox-rrd: use a journal to reduce amount of bytes written

proxmox-rrd: use a journal to reduce amount of bytes written

Append pending changes in a simple text based format that allows for
lockless appends as long as we stay below 4 KiB data per write.

Apply the journal every 30 minutes and on daemon startup.

Note that we do not ensure that the journal is synced, this is a
perfomance optimization we can make as the kernel defaults to
writeback in-flight data every 30s (sysctl vm/dirty_expire_centisecs)
anyway, so we lose at max half a minute of data on a crash, here one
should have in mind that we normally expose 1 minute as finest
granularity anyway, so not really much lost.

Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>