feat(audit): implement composite record id

Summary

• feat(audit): implement composite record id

  Logically, the record id consists of 3 fields:

  • raft_id -- raft id of the current node (or null if uninitialized);
  • count -- in-memory id which is auto incremented for each new record;
  • gen -- persistent id which is incremented each time node is started.

  However, in practice they are stored inside a global LogicalClock.

  This is what the record looks like now:

  {
    "id": "1.0.3",
    "instance_id": "i1",
    "message": "a new instance `i1` joined the cluster",
    "raft_id": "1",
    "severity": "low",
    "time": "2023-11-29T17:07:36.524+0300",
    "title": "join_instance"
  }

  Note how those fields are concatenated using .; the resulting unique id is represented by a json string type.

  Furthermore, since audit record logger now depends on the raft machine's state, we decided to move its initialization further into the init sequence (postjoin).

Part of #349 (closed)

Ensure that

• New code is covered by tests (still tbd later)
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if necessary) A follow-up doc issue: docs#134 (closed)