Add index to reservations.uuid column

Committing or rolling back reservations gets slow once a large number of
reservations have built up in the db.  Since they're being queried by
uuid this adds an index to speed up that operation.

Note that this can be slow with a large number of rows.

bug 1203872

Change-Id: Ida503206194a567a5c6811a304154d067c5faa83
Merge "Add index to reservations.uuid column"