Implement liveliness support

[sashacmc]

Implement liveliness support, can be enabled/disabled by means of Z_FEATURE_LIVELINESS

• Token:

  • z_liveliness_declare_token
  • z_liveliness_undeclare_token

• Subscriber: (require Z_FEATURE_SUBSCRIPTION enabled)

  • z_liveliness_declare_subscriber

• Query: (require Z_FEATURE_QUERY enabled)

  • z_liveliness_get

• Token interest query processing

Examples:

• z_sub_liveliness
• z_get_liveliness
• z_liveliness

Additionally:

• Implemented iterator for _z_int_void_map
• Cleaned up (reused) stuff from pull subscribers ( _remote_subscriptions)

Closes: #621

[github-actions]

PR missing one of the required labels: {'bug', 'dependencies', 'new feature', 'internal', 'documentation', 'enhancement', 'breaking-change'}

[OlivierHecart]

Another general comment:
As far as I understood, disconnection is not really managed yet run pico. But theoretically (and in Rust) when connectivity is lost, all remote tokens should be undeclared: an active liveliness subscriber should receive DEL messages for all alive remote tokens, and a liveliness get performed after the disconnection should get no replies.
Not sure if we want to handle that in this PR or another one.

[sashacmc]

Another general comment: As far as I understood, disconnection is not really managed yet run pico. But theoretically (and in Rust) when connectivity is lost, all remote tokens should be undeclared: an active liveliness subscriber should receive DEL messages for all alive remote tokens, and a liveliness get performed after the disconnection should get no replies. Not sure if we want to handle that in this PR or another one.

Yes, make sense, I created an issue to don't forget do it after I rework disconnection handling in #767