Fix closing DB which leaves the database locked

[hrmhatef]

What was the problem?

This PR resolves #78

How was it solved?

• Refactor the DB to stores an option of DB and change it to None in close function.

How was it tested?

• Add a new unit test to check the new implementation.
• All tests passed.

[github-actions]

Commit SHA:dcc7d5eacd955a2f28068618f39ae62c4e57949e

Test coverage results 🧪

Code coverage diff between base branch:main and head branch: 78_closing_db_with_release_lock 

Status	File	% Stmts	% Branch	% Funcs	% Lines
🟢	total	84.91 (1.64)	67.17 (1.53)	93 (2.1)	84.81 (1.65)
🟢	database.js	80.16 (3.31)	61.22 (2.04)	86.15 (4.62)	80.16 (3.31)
🟢	state_db.js	89.14 (0.77)	70.31 (1.56)	98.33 (0)	89.14 (0.77)