Make block placements remove ghost snitches

[awoo-civ]

Use case:

1. I place a snitch.
2. Somebody else destroys it.
3. The snitch outline is still there.
4. There's nothing I can do about it.

This change fixes that - I can now place a block where the snitch was to clear the outline.

May not be the best idea:

1. Snitch lookups are kinda slow (linear traversal of a generic collection) and we're doing it on every block placement.
2. This issue could be solved by deleting snitches not present in /jalist.

[Gjum]

I'm merging this but will make some changes: a98c6c5

• mark snitch as gone instead of deleting the entry
• find existing snitch by worldpos instead of iterating all known snitches