Make flushing_disabled() behave correctly when setting is unset.
Make flushing_disabled() behave correctly when setting is unset.

Previously, if no indexes had the disable_flush option set to any
value at all (i.e. get_index_translog_disable_flush() only returned
'unknown' values), flushing_disabled() would return 'enabled'
instead of 'unknown' because of the fallback when there was only
one unique state value. This commit rewrites that logic so that
it's easier to follow, if a bit less efficient.

anchor/elasticsearchadmin