Try to reduce memory pressure for Trac instance

[zooey]

Our Trac instance has a tendency to use a lot of memory When there's load on it.

This is partly due to Apache's mpm_prefork module being in use. We should investigate if it can be replaced by mpm_worker or mpm_event.

Additionally, a considerable part of the memory is used by the Postgresql processes serving the DB-connections. As Postgresql apparently doesn't seem to provide a threaded server model, we should investigate if the number of Postgres processes can be reduced without much impact on the performance.

When high load leads to memory pressure, the VM becomes unresponsive rather quickly, so I guess living with a DB using only few processes is better than leaving things like they are now ...