Introduce db connection max pool size configuration

How many connections used in the pool size could be dynamic in order to
adapt to any environment. This value can now be set by using
DB_CONNECTION_POOL_MAX_SIZE.

src/config.rs

@@ -333,6 +333,9 @@ make_config! {
         /// Log level
         log_level:              String, false,  def,    "Info".to_string();
 
+        /// Database connection pool max size.
+        db_connection_pool_max_size: u32,true,  def,    10;
+
         /// Enable DB WAL |> Turning this off might lead to worse performance, but might help if using bitwarden_rs on some exotic filesystems,
         /// that do not support WAL. Please make sure you read project wiki on the topic before changing this setting.
         enable_db_wal:          bool,   false,  def,    true;

src/db/mod.rs

@@ -42,8 +42,11 @@ pub mod schema;
 /// Initializes a database pool.
 pub fn init_pool() -> Pool {
     let manager = ConnectionManager::new(CONFIG.database_url());
-
-    r2d2::Pool::builder().build(manager).expect("Failed to create pool")
+    println!(CONFIG.db_connection_pool_max_size())
+    r2d2::Pool::builder()
+        .max_size(CONFIG.db_connection_pool_max_size())
+        .build(manager)
+        .expect("Failed to create pool")
 }
 
 pub fn get_connection() -> Result<Connection, ConnectionError> {