Add golang to dev VM

For $reasons.

We will be building binaries in CI, but want the runtime on the VM.
Merge pull request #573 from alphagov/feature/golang

Add golang to dev VM
Add PostgreSQL replication lag to collectd

Create a stored procedure to calculate the number of bytes secondaries
are lagging from the primary. This is only tracked on the primary so it
is only added to the collectd query list there.
Merge pull request #574 from alphagov/postgres-replication-monitoring

Add PostgreSQL replication lag to collectd
Initial draft of NTP monitoring

We have fabric tasks now to correct this, so let’s see how bad it is.
Merge pull request #575 from alphagov/feature/ntp-monitoring

Initial draft of NTP monitoring