Implement audit log (initial patch)

Summary

Implement ACL audit events

Currently, this implements some of the required audit events. See: https://docs.google.com/document/d/1wNXBtcVfzgawNb7ylvi8AQr3A4IxyseLmO5n1knWsOA

Implemented:

• features:

  • FFI wrappers for say.{h,c}
  • safe logger object on top of FFI definitions
  • unified tlog/audit definitions for formatting
  • fixed a needless allocation in tlog (new root per each line)

• events:

  • create_user
  • drop_user
  • create_role
  • drop_role
  • change_password
  • grant_privilege
  • revoke_privilege
  • create_table
  • drop_table
  • grant_role
  • revoke_role

Todo:

• features:

  • unique id generation
  • security subject (who executed the action)
  • audit configuration (via pico.audit function)

• well-defined events:

  • create_database (universe)
  • change_config
  • change_current_grade

• special events (local to each node):

  • auth_ok / auth_fail
  • access_denied (still not sure if we absolutely need that)
  • audit_rotate

• Unavailable (haven't been defined yet):

  • rename_user
  • create_procedure
  • drop_procedure

Related: #349 (closed)

Ensure that

• New code is covered by tests (tbd in follow-up MRs)
• API is documented
• Changelog is up to date (tbd in follo-up MRs)
• (if Lua API changed) Lua API version is bumped in luamod.rs
• A follow-up doc issue: docs#105 (closed)