Make the log output less chatty.

This includes the following changes:

- Log messages about adding a DNS record are shown only if the record
  does not exist already and log messages about removing a DNS record
  are shown only if the DNS record exists. This avoids unnecessary log
  chatter when working with static DNS records defined on the global
  level in a Vagrantfile that contains multiple VMs. Previously log
  messages about those static DNS records would have been shown every
  time a VM was started, now they are only shown when starting the first
  VM.

- Just like most log messages already have been, now log output from the
  DNS resolver logic is prefixed with "[landrush] " as well.