Change the way we store Enrollments

[Geod24]

Currently we add Enrollment to the EnrollmentManager when they are received but before they are validated:
https://github.com/bpfkorea/agora/blob/05bfe591aec457544bfe9fc8b799f592d4d6b303/source/agora/node/Node.d#L507-L518

Then consensus is made on them. This has a few issues:

• It is impossible to replace an enrollment, lest the user change their key (so there's a 2 week waiting period);
• This table is never cleaned up;
• We mix the data we use before and after consensus, which is error-prone.
• addValidatedBlock does not add Enrollments, only update their height, which mean catch-up is broken.

It would be best to have those two data set apart, and only have active enrollments in the EnrollmentManager. The separation should probably be similar to transaction pool / UTXO set.

Definition of Done:

• Make the way to sotre Enrollments like the way similar to to transaction pool / UTXO set.
• Clean up the table when a enrollment is expired.

[linked0]

Among the issues to be resovled, some issues as follows can be split into another issue.

• It is impossible to replace an enrollment, lest the user change their key (so there's a 2 week waiting period);
• This table is never cleaned up;

Those can be handled after chaning the way we store the Enrollments. So I will create new issue named like "Add the feature to change or delete enrollments stored".

[linked0]

After the PR #748 , there will be more PRs for tasks as follows.

• Refactor the relations between classes related to the enrollment process and other classes that use the classes(including the change of the name and function modifier of member functions.
• Refine the API endpoints related to the enrollment process, for example, getPreimage.
• Remove table in the EnrollmentManager.
• Refine test code in the test.EnrollmentManager package.

[Geod24]

@linked0 : I think we can reasonably consider this closed. Could you open issues for any remaining tasks so they can be addressed in the next sprint ?

[linked0]

@linked0 : I think we can reasonably consider this closed. Could you open issues for any remaining tasks so they can be addressed in the next sprint ?

There is one remaining task which is to remove the table from the EnrollmentManager. I will create an issue and address it in the next sprint.