make each db backend responsible for its own setup

RocksDB automatically creates parent directories already. SQLite doesn't
create parent directories, only the db file if it didn't exist.

src/database.rs

@@ -3,7 +3,6 @@ use std::{
     fs,
     io::Write,
     mem::size_of,
-    path::Path,
     sync::Arc,
 };
 
@@ -241,16 +240,6 @@ impl KeyValueDatabase {
             "Compiled without support for any databases",
         ));
 
-        if !Path::new(&config.database.path).exists() {
-            fs::create_dir_all(&config.database.path).map_err(|_| {
-                Error::BadConfig(
-                    "Database folder doesn't exists and couldn't be created \
-                     (e.g. due to missing permissions). Please create the \
-                     database folder yourself.",
-                )
-            })?;
-        }
-
         let x: Arc<dyn KeyValueDatabaseEngine> = match config.database.backend {
             #[cfg(feature = "sqlite")]
             DatabaseBackend::Sqlite => {

src/database/abstraction/sqlite.rs

@@ -1,5 +1,6 @@
 use std::{
     cell::RefCell,
+    fs,
     future::Future,
     path::{Path, PathBuf},
     pin::Pin,
@@ -67,6 +68,8 @@ pub(crate) struct Engine {
 
 impl Engine {
     pub(crate) fn open(config: &Config) -> Result<Self> {
+        fs::create_dir_all(&config.database.path)?;
+
         let path = Path::new(&config.database.path).join("sqlite.db");
 
         // calculates cache-size per permanent connection