{WIP} Implement on_primary hooks.

[DimCitus]

This allows users of pg_auto_failover to setup their own scripts/actions to complement a failover. The hooks are run in a process that is separate from the main FSM, so as to prevent the system from making progress.

As a result, it's not possible for the hooks to change how things are implemented in pg_auto_failover itself.

The hook system also allows running a user-defined "service", which is a long running process or a deamon that belongs to pg_autoctl process tree.

• [x] Implement a new internal service for running user-defined hooks
• [ ] Implement pg_autoctl enable|disable run-hooks
• [ ] Implement pg_autoctl create listener
• [ ] Implement support for running script/commands (man system)
• [ ] Add unit testing support for user-defined hooks
• [ ] Document the new hook system, including tutorial
• [ ] Add documentation examples covering pgbouncer as a hooked system

Fixes #878. Closes #573. Closes #539. Closes #508. Fixes #490. Fixes #440. Fixes #427.

[abubakarm94]

This feature will be extremely valuable. Any thoughts on when we can expect this?