Return error if operation is attempted on a closed session

[StephenCathcart]

A session is still usable even after closing it explicitly:

session := driver.NewSession(ctx, sessionConfig)
session.Close()
// These all still work
session.Run(...)
session.BeginTransaction(...)
session.ExecuteRead(...)
session.ExecuteWrite(...)

We now return a new UsageError when an operation is attempted on a closed session.

Fixes issue: #569 raised by @oleriajm

[StephenCathcart]

Session is still usable even after explicitly closing it