Add timeout to read from channel in GetBlobOperation

Currently, a `GetBlobOperation` will not be completed when the user does not end up reading from `GetBlobResult`. A possible solution is to add a timeout that completes the operation if the user does not read from `GetBlobResult` in a reasonable timespan.

Start by reading GetBlobOperation and GetBlobResult to trace how completion depends on the result channel being read. Determine the appropriate timeout behavior for an unread result, then verify that the operation completes after the timeout without disrupting normally consumed results.

Written by the indexing model from the issue text.