Deprecate Database::flush() function

The Database::flush() function is only needed for the sled database on mobile, instead for mobile use the sqlite database.

src/database/any.rs

@@ -61,6 +61,7 @@ macro_rules! impl_from {
 
 macro_rules! impl_inner_method {
     ( $enum_name:ident, $self:expr, $name:ident $(, $args:expr)* ) => {
+        #[allow(deprecated)]
         match $self {
             $enum_name::Memory(inner) => inner.$name( $($args, )* ),
             #[cfg(feature = "key-value-db")]

src/database/mod.rs

@@ -159,6 +159,10 @@ pub trait Database: BatchOperations {
     /// It should insert and return `0` if not present in the database
     fn increment_last_index(&mut self, keychain: KeychainKind) -> Result<u32, Error>;
 
+    #[deprecated(
+        since = "0.18.0",
+        note = "The flush function is only needed for the sled database on mobile, instead for mobile use the sqlite database."
+    )]
     /// Force changes to be written to disk
     fn flush(&mut self) -> Result<(), Error>;
 }