feat: make session snapshot more full proof

[soumyamahunt]

1. Implemented transacted write model for creating session snapshot files (both backup and metadata file).
2. Reduced number of IO operations to improve speed during load.
3. Implemented recovering tabs from orphaned snapshot files.

PR Type

What kind of change does this PR introduce?

• Feature

[soumyamahunt]

@Jasonstein I think notepads should use database like Realm instead of using a json file to store metadata. This would make storing metadata safer. Also, we can use .~TMP files remaining from failed write transaction to recover more data.