Add validation of IP addresses and domains in settings

Many settings in ClickHouse's configuration file specify hosts in the form of IP addresses (`12.13.14.15`) or domains (`mail.stanford.edu`). Example:

```xml

172.31.0.47
...

```

Many similar settings exist. As of now, these settings are parsed as strings and typos (e.g. a leading space in above example) cannot be detected immediately - in this case the connection to the host would silently fail. We should add a method `bool getHost(const std::string & key) const;` to "base/poco/Util/include/Poco/Util/AbstractConfiguration.h" which does some basic validity checking by itself and throw an exception accordingly.