Define client.reserved
This patch adds client.reserved attribute to Client class.
client.reserved reflects the state of its associtated environment's
execution ready flag. When an environment is initially created (not
fully ready with the environment settings object/global object/fetched
main resource), this boolean attribute returns true. When the
environment's execution ready flag is set (the environment settings
object is created and the main resource is fetched), client.reserved
returns false.
