Refactor Source interface to allow/prepare for extensibility

[anandthakker]

Replaces #2582

• Inverts TilePyramid and Source dependencies. TilePyramid now essentially serves as a common wrapper class for dealing with sources.
• Source types are now essentially:

(
  name: string,
  createSource: (id, options, dispatcher, onChange, callback: (err, source: Source) => void) => void
  ?workerSourceURL: string
)

• And a Source is an object implementing:

id: string // <- it's a little weird to rely on sources presenting this as a property
tileSize: number
minzoom: number
maxzoom: number
// not sure about these three properties
roundZoom: boolean
reparseOverscaled: boolean
isTileClipped: boolean

load: (tile: Tile, cb: (err, ?data) => void) => void
abort: (tile: Tile) => void
unload: (tile: Tile) => void
serialize: () => Object // plain JS

Various todo's and questions:

• The Tile objects are used by various source types to maintain tile-specific state; ideally, this implicit contract between TilePyramid and Source should be made explicit in the Source interface.
• Decide on an approach to serialize() (see note here)
• Source.is is only used in one place: to support passing already-created sources to addSource (and prevent the attempt to validate against the spec in that case). What's the best way to do this in the factory function approach, where it's a little weird to rely on instanceof?
• Map methods (addSource, getSource). Related to this and the previous item: is it important to retain the ability to create sources without adding them to the map? If so, it might make the most sense to expose a createSource on the map obj, since creation now depends on a reference to dispatcher, which probably shouldn't be exposed.
• It's weird to rely on sources presenting an id property with the correct value.
• Reinstate query*Features
• I'm not sure what the data yielded by load should look like. Currently, the only thing I can see being needed by code outside of the particular source is bucketStats, which the pyramid uses to emit the tile.stats event.

[anandthakker]

@lucaswoj this could also be (name, { create: (id, options, dispatcher, onChange, callback) => void, ?workerSourceURL }, callback). Any preference between the two signatures?

[anandthakker]

➡️ #2667