Add index for reservations on (deleted, expire)

the query for expire_reservations currently does a full table scan.
This adds an index so frequent invocations of expire does not bog
down the database.

Change-Id: I82c6064159d7bfa7f4dc7fca5a7c9bc52b0f07fb
Resolves-bug: 1348720
Merge "Add index for reservations on (deleted, expire)"