fix: sewer rework

[Vollch]

Purpose of change

Mostly to remove sewers generation from generate_sub(). To eventually(after mutable mines) remove whole that loop iterating over all overmap tiles.
And it also fixes few sewers-related bugs, such us when sewer entrance is disconnected from tunnel(due to missing end mapgen), or when sewer leads to the point with no entrance at all(due to lack of manhole in city center).

Describe the solution

Sewer mapgens combined in one function, to implement mapgen for end tiles, and reduce clutter.
Roads used to override manholes due to terrains order in overmap_connections.json, fixed now. Chance to generate manhole reduced from 1/2 to 1/8 to compensate increased amount.
City centers now have manholes - sewer specials connects to center, so it need to way in. Also, all manholes generate subways during city generation, so it won't need to iterate whole map searching for manholes later on.
Each town have its own isolated sewer, with no connections between cities. Such connections makes some sense for subways, but definitely not for sewers. And it's not very fun if sewers and subways would had basically same layout.

Describe alternatives you've considered

Building sewers connections right in build_city_street(). But this way it would steal undergrounds from potential basements, it's safer to juggle a vector around.
Porting jsonified sewer from DDA. That's... an option. But replacing 20 lines of code wth 1000 lines of json is... meh. I'd rather delegate it to lua eventually.
Joining sewers of overlapping cities. Still considering.

Testing

Generated bunch of overmaps.

Additional context

Sewers of different cities:

There's no cut tunnels, all of them ends with entrance of some kind(specials, manholes).

[scarf005]

seems to work correctly, LGTM.