Add a timeout for forceMergeDeletes in IndexWriter

### Description

Using IndexWriter's `forceMergeDeletes` to eliminate merge debt is a very useful feature -- especially during initial indexing. However, larger indexes can require 20+ minutes to finish a `forceMergeDeletes` call. It would be a useful feature if we add an API for specifying a timeout for the existing `doWait` argument. Instead of blocking until `forceMergeDeletes` finishes, we should block until either `forceMergeDeletes` finishes or the timeout is reached, whichever comes first.

Start at IndexWriter's forceMergeDeletes API and trace how its existing doWait argument waits for completion. Determine where a timeout can be specified and how the call should behave when the timeout is reached. Done means the API returns after completion or timeout without blocking indefinitely, with behavior covered by the relevant tests.

Written by the indexing model from the issue text.