This commit introduces a hugepages configuration for postgres via a
rhizome lib+bin.
At the moment, it uses the default huge page size (2M), and calculates
the number of huge pages as 25% of the total available memory. The setup
adjusts the shared_buffers accordingly to fit the shared memory within
the allocated number of hugepages.
The configuration happens as an `ExecStartPre` script added to the
existing systemd unit managed by pg_ctlcluster. To keep the semantics of
the `start` command unchanged, the hugepages script *does not* stop an
existing cluster to configure hugepages, and only does it when the
cluster is not running.
In a subsequent change, we can consider adding the boot parameters to
setup 1G hugepages for larger instances (possibly standard-30+), where
they are most useful.
[1]: https://www.postgresql.org/docs/current/kernel-resources.html#LINUX-HUGE-PAGES
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