Wrong static clear on taxonomy_vocabulary_save()

Found out when working on the field info cache optimization in #1040790: _field_info_collate_fields() memory usage :

When a vocab machine name gets changed, there can be a race condition where entity_get_info() stores stale data and still lists the old bundle instead of the new one :

- taxonomy_entity_info() relies on taxonomy_vocabulary_get_names() to fetch the list of 'taxo terms' bundles (= vocabs).
That function queries the {taxonomy_vocabulary} table and caches the results in a static var.

- when creating / updating a vocab, taxonomy_vocabulary_save()
writes to {taxonomy_vocabulary},
calls field_attach_[create|rename]_bundle(), which clears the entity_info() cache
then calls a couple hooks (hook_entity_[insert|update]...),
and only then clears the taxonomy_vocabulary_get_names() static cache.

--> If anything calls entity_get_info() in between (and field_sql_storage_field_attach_rename_bundle() does, through field_read_instances()), then entity_get_info() gets rebuilt based on the stale static list of vocabs, and primes its own static + persistent caches with it.

Patch coming up.