Implement "watch" mode for "cartridge start"

[knazarov]

When I develop code, I would like "cartridge start" to automatically watch for changes on filesystem and reload the code.

The primitive approach may be to check the code for errors by doing a "load", and then doing a complete restart in the same session if the code loads OK.

More advanced approach may be to do hot code reload by reloading modules in package.loaded.

[artur-barsegyan]

Depends on tarantool/cartridge#1100