Adding DRAINING state into api.thrift.

Splitting this work into 2 steps to ensure schema compatibility between releases.
Step1: Adding DRAINING state into api.thrift and rewriting DRAINING to RESTARTING in StorageBackfill.

To come:
Step2: Implement support for DRAINING in state machine.

Testing Done:
gradle clean build

Reviewed at https://reviews.apache.org/r/17666/
Adding PGP key for maxim@apache.org

Reviewed at https://reviews.apache.org/r/17676/