reorganize database crate.

split database Cork into unit.

split database migrations from mod.rs

Signed-off-by: Jason Volk <jason@zemos.net>

src/database/kvengine.rs

@@ -0,0 +1,38 @@
+use std::{error::Error, sync::Arc};
+
+use super::{Config, KvTree};
+use crate::Result;
+
+pub(crate) trait KeyValueDatabaseEngine: Send + Sync {
+	fn open(config: &Config) -> Result<Self>
+	where
+		Self: Sized;
+
+	fn open_tree(&self, name: &'static str) -> Result<Arc<dyn KvTree>>;
+
+	fn flush(&self) -> Result<()>;
+
+	#[allow(dead_code)]
+	fn sync(&self) -> Result<()> { Ok(()) }
+
+	fn cork(&self) -> Result<()> { Ok(()) }
+
+	fn uncork(&self) -> Result<()> { Ok(()) }
+
+	fn corked(&self) -> bool { false }
+
+	fn cleanup(&self) -> Result<()> { Ok(()) }
+
+	fn memory_usage(&self) -> Result<String> {
+		Ok("Current database engine does not support memory usage reporting.".to_owned())
+	}
+
+	#[allow(dead_code)]
+	fn clear_caches(&self) {}
+
+	fn backup(&self) -> Result<(), Box<dyn Error>> { unimplemented!() }
+
+	fn backup_list(&self) -> Result<String> { Ok(String::new()) }
+
+	fn file_list(&self) -> Result<String> { Ok(String::new()) }
+}