config: adjust max_filters calculation to reflect current memory usage
Change max_filters calculation for systems with recommended memory
setup (>4GB).
The values of 2816 and 150 are based on testing with 4GB, 6GB & 8GB
memory configurations, large and small numbers of added objects and
sending multiple mails simultaneously.
On setups with less memory, it is difficult to completely prevent OOM kills.
So for these setups the calculation remains similar, but a warning is sent.
Related OOM killer problem found in forum:
https://forum.proxmox.com/threads/123531/