logical_replication_mode

I suggest we rename this setting to something starting with debug_. 
Right now, the name looks much too tempting for users to fiddle with.  I 
think this is similar to force_parallel_mode.

Also, the descriptions in guc_tables.c could be improved.  For example,

     gettext_noop("Controls when to replicate or apply each change."),

is pretty content-free and unhelpful.