Provide more headroom for unbounded idle connections

[MikeTheCanuck]

As a temporary workaround, the database will need to accommodate more long-lived idle connections for some of the APIs that haven't implemented a connection pooling or idle-connection-killing strategy yet (see #177 and #178).

The first step is to increase max_connections. However, that will require additional measures: https://stackoverflow.com/questions/30778015/how-to-increase-the-max-connections-in-postgres#32584211

And we risk seeing available memory on the DB server drop precipitously as we pursue this kind of workaround.

[MikeTheCanuck]

Changes in /data/postgresql/databases/postgresql.conf:

• max_connections = 200 (from 100)
• shared_buffers = 256MB (from 128MB - double the connections, double the buffers)

Changes in /etc/sysctl.conf:

• kernel.shmmax=301989888 (288MB)l

Then we rebooted the postgresql instance by running the following:

sudo systemctl stop postgresql
sudo systemctl start postgresql