Add a `TransportConfig' class which should contain configuration options (such
as the state location) meant for pluggable transport modules.
Use the new `TransportConfig' class to store the state location and pass it the
`TransportConfig' object into the pluggable transport's constructor.
Update the constructors of all existing four transports to expect a
`transport_config' variable which is an instance of the new `TransportConfig'
class.
Add ChangeLog entry for #9815.

(And specify the name of an optional argument in a function call.)