Default `ADDON_TREE_CACHING` to `cacheKeyForTree`.
Default `ADDON_TREE_CACHING` to `cacheKeyForTree`.

The proposed public API for this is `cacheKeyForTree`, with this 
change when the symbol is removed (i.e. the feature is stable) a
new version of ember-engines will not need to be released (and
users will not accidentally fall back to uncached builds).

Specific scenarios:

* In older versions of ember-cli where `ember-cli/lib/experiments`
is not present, we will not add the caching method at all.

* In versions of ember-cli with `ember-cli/lib/experiments` AND
`ADDON_TREE_CACHING` symbol in the experiements file:
that symbol will be used to name the method.

* In versions of ember-cli with a `ember-cli/lib/experiments`
file that does not contain the `ADDON_TREE_CACHING`
symbol in the experiments file (i.e. it has been promoted
to a full public API) the method will be named 
`cacheKeyForTree`.
ember-engines/ember-engines,dgeb/ember-engines,ember-engines/ember-engines,dgeb/ember-engines