Retain agent draining start time in master

The master should store in memory the last time that a DrainSlaveMessage was sent to the agent so that this time can be displayed in the web UI. This would help operators determine the expected time at which the agent should transition to DRAINED.

We should update the webui to use that time as a starting point and the DrainConfig's max_grace_period to calculate the expected maximum time until the agent is drained.