feature: two-step plugin installing and removing

Now for installation and enable plugin, there are two steps instead of one
(`pico.load_plugin` previously).
1. Install plugin (`pico.install_plugin`):
validate plugin, create system tables.
2. Enable plugin (`pico.enable_plugin`):
loads plugin into instances memory, call `on_start` callback.

Same for a plugin removing process (`pico.remove_plugin` previously):
1. Disable plugin (`pico.disable_plugin`): call `on_stop` callbacks, drop
plugin from instances.
2. Remove plugin (`pico.remove_plugin`):
remove plugin meta from system tables.