Using get_term_by() rather than direct SQL queries to fetch the alias term
fixes a number of issues:
* Object cache for aliased term is properly cleared after update.
* If the aliased term is in the object cache, it's served from there, saving a database query.
* Duplicate 'edit_terms' and 'edited_terms' hooks can be removed.
* Fix a PHP notice when the 'alias_of' term is not found.
* Prevent the incorrect creation of a new term group for the primary term when the 'alias_of' term is not found.
Adds unit tests for 'alias_of' functionality in both functions.
Fixes#29848.
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git-svn-id: http://core.svn.wordpress.org/trunk@29622 1a063a9b-81f0-0310-95a4-ce76da25c4cd