Apply Singleton pattern to EuTxManager

[SeonJK]

AS-IS

While using EuTxManager with multiple instances, they cause wrong execution.
For example, adding multiple EuTxManager due to add multiple EuPI, app cannot easily manage EuPI.

TO-DO

By applying Singleton pattern, code prevents from several issues.

I suggest singleton pattern by Bill Pugh solution. It makes sure Thread-safe.

Applying singleton pattern by @volatile and @synchronized causes performance decreasement.
However by Bill Pugh solution, EuTxManager class makes singleton through Holder class. Holder class makes sure to load once.

References:

• https://stackoverflow.com/questions/50771372/bill-pugh-singleton-why-thread-safe
• https://itzone.com.vn/en/article/bill-pugh-singleton-in-java-incredibly-simple/
• https://medium.com/swlh/singleton-design-pattern-with-kotlin-2e6c8d42fc11#:~:text=Singleton.INSTANCE.doSomeThin()-,we,-can%20create%20a

[yeonns]

@SeonJK Thank you for suggestion! Great diagram and description 👍
If you apply singleton to EuTxManager, it will be very helpful!