Unify Khepri paths API

[dumbbell]

Why

Currently, rabbit_db_* modules use and export the following kind of functions to return the path to the resources they manage:

khepri_db_thing:khepri_things_path(),
khepri_db_thing:khepri_thing_path(Identifier).

Internally, khepri_db_thing:khepri_thing_path(Identifier) appends Identifier to the list returned by khepri_db_thing:khepri_things_path(). This works for the organization of the records we have today in Khepri:

|-- thing
|   |-- <<"identifier1">>
|   |   <<"identifier2">>
`-- other_thing
    `-- <<"other_identifier1">>

However, with the upcoming organization that leverages the tree in Khepri, identifiers may be in the middle of the path instead of a leaf component. We may also put other_thing under thing in the tree.

That's why, we can't really expose a parent directory for thing and other_thing. Therefore, khepri_db_thing:khepri_things_path/0 needs to go away. Only khepri_db_thing:khepri_thing_path/1 should be exported and used.

In addition to that, there are several places where paths are hard-coded (i.e. their definition is duplicated).

How

The patch does exactly that. Uses of khepri_db_thing:khepri_things_path() are generally replaced by rabbit_db_thing:khepri_thing_path(?KHEPRI_WILDCARD_STAR).

Places where the path definitions were duplicated are fixed too by calling the path building functions.

In the future, for a resource that depends on another one, the corresponding module will call the rabbit_db_thing:khepri_thing_path/1 for that other resource and build its path on top of that.