feat: `RoleTable` trait and three implementations

[waynexia]

Which issue does this PR close?

ref #175

Rationale for this change

Following #175, this patch defines the trait RoleTable and implement three roles on top of it.

What changes are included in this PR?

• Define RoleTable
• Draft LeaderTable, fill two methods write and flush as examples. They are two ways to organize code.

Are there any user-facing changes?

TBD, probably no

How does this change test

TBD