[#121] client: Make PrmObjectGet/Head/GetRange fields public

* Remove common PrmObjectRead structure
* Introduce buildRequest for PrmObjectGet/Head/GetRange
* Refactor the usage of these params in pool

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

client/common.go

@@ -47,6 +47,8 @@ func writeXHeadersToMeta(xHeaders []string, h *v2session.RequestMetaHeader) {
 		return
 	}
 
+	// TODO (aarifullin): remove the panic when all client parameters will check XHeaders
+	// within buildRequest invocation.
 	if len(xHeaders)%2 != 0 {
 		panic("slice of X-Headers with odd length")
 	}