fix: drop picodata spaces with granted privileges

Summary

• fix: drop picodata spaces with granted privileges

Previously picodata failed to drop spaces with grants on it ("the space has grants"). Another problem was that we didn't clean _pico_privileges grants when the space was dropped. Both problems were fixed in the current commit.

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if API docs changed) A follow-up doc issue is created in picodata/docs and linked here

Closes #355 (closed)