datastore.c: Improve error handling and docs.

datastore.c: Improve error handling and docs.

 - Add Doxygen comments for all functions (some TODOs remain).

 - Check for invalid input parameters (such as NULL pointers etc).

 - Return SR_ERR_ARG upon invalid input parameters.

 - Make sr_datastore_put() return int instead of void, so we can pass
   an error code (SR_OK, SR_ERR_MALLOC, and so on) to the caller.