Fix compile warnings in mantis-server-agent module

[liuml07]

There are compile warnings in the code due to:

• deprecated APIs
• missing @Nullable/@NotNull annotations
• incomplete javadocs
• lambda can be replaced with method references
• raw usage of generic types
• unnecessary type parameters (like Map<String> m = new HashMap<String>())
• anonymous class can be replaced with lambdas and more.

We don't need to solve all of them, but a good coding exercise is to address most those compile warnings when possible. Guidelines are:

1. Not change code logic, which should be addressed and filed PR separately
2. Not change public interfaces (class, methods and fields) that would break other modules or applications. But it's safe to rename private fields or methods when necessary.
3. Prefer readability to more concise code
4. Improve tests in the module is welcome but not required
5. Compile and pass all existing tests.

This tracks the effort for the mantis-server-agent module to make the code change scoped.

[7ue5wu]

Hi @liuml07 can you please assign this task to me?

[nick2432]

can i work on this?