Suggestion: Encapsulate usage of ets tables

[peterzeller]

We have a few places in the Antidote codebase that make use of ETS tables (search for ets:). It is not easy to see at a glance what is stored in the tables and whether any processes use the same table as shared memory.

For example: clocksi_readitem_server:get_active_txns_key_internal passes a table-id to clocksi_vnode:get_active_txns_key and it is not directly clear who owns the table.

I suggest to refactor this such that we have a separate module for each table that has a name / can be used in multiple modules. The module should have a clear API such that it is clear what is stored in the table.
Instead of passing around table names (atoms) or table-ids we could then use different custom types for the different tables.