Remove unnecessary &mut from begin_transaction()

Transaction::new() only needs &Database (not &mut) because:
- We only call db.clone() which doesn't mutate the Database
- All transaction operations go through the C API via get_ref()
- This makes the API more flexible - no need for mutable Database

This allows usage like:
let db = Database::open("test", None)?;  // immutable
let transaction = db.begin_transaction()?;  // works now

Author: TonyMasson

src/database.rs

@@ -358,7 +358,7 @@ impl Database {
     /// it will be rolled back when dropped.
     ///
     /// For simpler cases, consider using `in_transaction()` instead.
-    pub fn begin_transaction(&mut self) -> Result<Transaction> {
+    pub fn begin_transaction(&self) -> Result<Transaction> {
         Transaction::new(self)
     }