Previously, the Force Merge API allowed the parameters only_expunge_deletes
and max_num_segments to be set to a non default value at the same time. But
the max_num_segments was silently ignored when only_expunge_deletes is set
to true, leaving the false impression that it has been applied.
The Force Merge API now rejects requests that have a max_num_segments greater
than or equal to 0 when the only_expunge_deletes is set to true.
The index.force_memory_term_dictionary setting was introduced in 7.0 as a
temporary measure to allow users to opt-out of the optimization that leaves the
term dictionary on disk when appropriate. This optimization is now mandatory
and the setting is removed.