cluster: add helpers to modify cluster config

[mandesero]

This patch adds the ability to keep and adjust the cluster declarative configuration with cluster:modify_config(), and then apply the changes later via cluster:apply_config_changes() without passing an explicit config.

Motivation

Instead of overloading cluster:sync(...), I decided to introduce a dedicated method :apply_config_changes(). This avoids awkward patterns such as:

• cluster:sync(nil, opts)
• cluster:sync(cluster:modify_config():<some-conf>:config(), opts)
• or other ad-hoc workarounds.

Usage example

local c = cluster:new(config, opts)
c:start()
-- <..do workload..>

c:modify_config()
    :set_global_option('replication.failover', 'manual')
c:apply_config_changes(<..maybe-opts..>)

-- <..do workload..>

Backward compatibility

The old API continues to work.
However, to avoid ambiguous states when using the new API, the following rules now apply:

• If modify_config() has been used and the changes were not committed via apply_config_changes(), calling:

  • c:sync(new_config, opts),
  • c:reload(new_config)

  will raise an exception.

• New c:config() returns the last applied configuration, so calling it while there is an uncommitted builder also raises an exception.

This ensures the cluster object never ends up in a half-modified state.

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That said, I’m open to discussion if this behavior doesn’t fully meet expectations for this feature.

Closes #426