[FlexiDag]Add read-write lock in dag db store

[jackzhhuang]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Other information

[nkysg]

maybe we don't need this drop. Please see this https://amanieu.github.io/parking_lot/parking_lot/struct.RwLock.html

Does not require any drop glue when dropped.
this is a demo from the above link.

use parking_lot::RwLock;

let lock = RwLock::new(5);

// many reader locks can be held at once
{
    let r1 = lock.read();
    let r2 = lock.read();
    assert_eq!(*r1, 5);
    assert_eq!(*r2, 5);
} // read locks are dropped at this point

// only one write lock may be held, however
{
    let mut w = lock.write();
    *w += 1;
    assert_eq!(*w, 6);
} // write lock is dropped here

[nkysg]

remove comment code?

[nkysg]

remove extra // ?