Perform definition import after plugins are enabled

[michaelklishin]

Currently there is a chicken-and-egg problem triangle between

• Default data seeding
• Definition import on boot
• Plugin activation

which can be problematic when definitions related to or dependent on plugins are involved, e.g. an exchange of a custom plugin-provided type.

We can potentially untangle this with a few low risk changes:

• Avoid default data seeding when load_definitions is configured
• Load definitions not in a core boot step but explicitly in a "post-run" phase, after plugins have been enabled
• As a bonus, consider enabling client connection listeners after all plugins have been enabled (but this can be problematic: consider a Shovel that has to connect to the local node upon startup)

While definitions can be imported at any point after node startup, it would be convenient for Shovel and custom exchange type users if this limitation of load_definitions was lifted.

See #2381 for the background.

[tbnguyen1407]

Hello, this seems still not working for x-consistent-hash exchange plugin when an exchange of that type is defined in definitions.

I removed the exchange definition and see the plugin bootstep is still performed after definition import

Can you help take a look?