Merge changes I301ab7d8,I66eda02e,I0d7cbe83,Ic37b58e6

* changes:
  Ensure no state changes after mDestroyed=true (4/n)
  Remove unused code (3/n)
  Merge streamValidateAndCommit() and streamAndValidateLocked() (2/n)
  Rewrite how we abandon stages sessions (1/n)