make possible to rewrite destination urls by domain mappings

[ziollek]

Rationale:
The current approach assumes that if the mapping exists for the destination URL domain then such domain should be replaced by source domain mapping. Such an assumption has no justification. The new approach is based on simple logic:

• if source and destination domains share the same mappings then the destination URL is replaced by matched source domain
• in another case if there exists exactly one mapping for the destination domain such mapping is used
• if any of the above conditions are met by the default destination domain it is used without any altering

What has been done

• changing init for VclRedirect object (we additionally pass there destination URL)
• adding helper methods that provide all mapping for the parsed destination domain
• implementing logic according to the rules described above