This also makes the CompiledRoute implement Serializable in order to:
1. make the serialization format shorter
2. have no null bytes in there, which the native serializer add for private properties, and thus would complicate saving in databases etc.
3. We should add to our symfony BC promise, that only classes that implement Serializable are ensured to be deserializable correctly with serialized representations of the class in previous symfony versions.
As explained in #6775, this has been done for the following reasons:
1. It's also Request::getHost()
2. The term hostname has been obsoleted in
http://tools.ietf.org/html/rfc3986#appendix-D.2 and uses the host only
3. hostname in the RFC was defined as the registered domain name, but we
probably also want to match IP-Adresses with the pattern which is the
host = IP-literal / IPv4address / reg-name for.
