currently, we treat all strings as shared (either by full equality or identity equality), while almost all times we know if they should be serialized as shared or not. Add an explicitly write/readSharedString, and use it where applicable, and all other write/readString will not treat them as shared
relates to #3322
introduce a new class, TransportRequest, which includes headers. This class can be used when sending requests over the transport layer, and ActionRequest also extends it now.
This is the first phase of the refactoring part in the transport layer and action layer to allow for simpler implementations of those as well as simpler "filtering" capabilities in the future
