Update namespaces for Meson Client, Scheduler, and Executor APIs

[oubiwann]

Given the latest changes to the codebase (the new protocols and behaviours and the associated reorganization), we need to update (and add) several namespaces, in particular the scheduler and executor APIs. These changes will allow the scheduler and executor APIs to better fit into the whole (in terms of mechanics, that is).

If we use the new approach, tasks would include:

• Add meson.client.impl.master.scheduler
• Add meson.client.impl.agent.executor
• Remove meson.client.ClientAPI
• Remove meson.client.Client
• Remove SchedulerAPI
• Remove SchedulerClient
• Clean up meson.client
• Move meson.client.base to meson.client.common and consolidate code

The following are partially done, but will be consistently added to over time:

• Add convenience functions in meson.scheduler namespace
• Add convenience functions in meson.executor namespace

Part of epic #2