Vacuum full connection exhaustion

Hey folks,

I noticed something weird, and not sure if this is the expected behaviour
or not in PostgreSQL.

So I am running Benchbase (a benchmark framework) with 50 terminals (50
concurrent connections).
There are 2-3 additional connections, one for a postgres-exporter container
for example.

So far so good, and with a `max_connections` at 100 there is no problem.
What happens is that if I execute manually `VACUUM FULL` the connections
are exhausted.

Also tried this with 150 `max_connections` to see if it just “doubles” the
current connections, but as it turned out, it still exhausted all the
connections until it reached `max_connections`.

This was cross-checked, as the postgres-exporter could not connect, and I
manually was not allowed to connect with `psql`.

Is this expected or is this a bug?

postgres-exporter logs:
```
sql: error: connection to server on socket
"/var/run/postgresql/.s.PGSQL.5432" failed: FATAL: sorry, too many clients
already
```