ipam: move mac-cache.db to unprivileged sdn/ subdirectory

ipam: move mac-cache.db to unprivileged sdn/ subdirectory

follows commit:
0f48bc6 ("ipam: move state file of PVE plugin over to common sdn directory")
as far as reasoning goes, and also closely code-wise (if only to make
the clean-up with PVE 9.0 a bit more straight-forward):
files in priv/ are sensitive in the sense that access there can be
used to hijack (external systems) - the mac-cache can be kept next to
the remaining sdn-config.

minimally tested on my machine.
depends on the pve-cluster commit sent with this.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>