v1.18.0

Add APIs for graceful drain (#15)

* Add APIs for graceful drain

This patch adds the following APIs for supporting graceful drain:
(1) Inputhost Admin - SealExtent() API to be called by the controller
    to start the graceful drain
(2) New InputHost command type to send DRAIN command to the clients.

Note: A couple of things:
* Since we are just using the SealExtent API, I just moved the SealRequest
  definition to shared.thrift and is used by both store and admin.
* Since now we include shared.thrift, there was a name collision with
  ListDestinations and so had to rename it to ListLoadedDestinations.

* Address CR comments

Use a new DrainExtents() API rather than using the already existing
SealExtent() API.

The new API can get a list of <destinationUUID, extentUUID> pairs to
drain all the respective extents gracefully.

* Make sure we generate files properly