CLI with async capabilities

[rwestphal]

Once #6371 is merged, FRR will have built-in support for asynchronous YANG-modeled operational data retrieval. As of now, however, the CLI can't take advantage of this new capability since all CLI commands are synchronous, they block the main pthread until they are finished.

It should be possible, however, to have asynchronous commands by doing some small changes in the CLI code. One possibility would be to add a new CLI return value (e.g. CMD_SUSPEND) which would instruct the CLI to stop reading from stdin. Then, at any given point, a new API function like vty_resume(vty) could be used to instruct the CLI to resume reading from stdin.

These small additions would allow us, for example, to write commands that fetch bulk data (e.g. millions of routes) in small chunks, without blocking the main pthread of the relevant daemons.

A slightly more advanced implementation would also keep track of the event (or thread) the async command is waiting on, and run a timeout timer to abort the command in case it's taking too long to complete. That would prevent the CLI from becoming unresponsive because of a badly behaved command.

I'm opening this issue mostly for tracking purposes and also to discuss alternate implementation ideas.