Clarify use cases for multitiles. How to limit request volume.

[joanma747]

These comments came out form the WMS.SWG Toulouse 2019 meeting:

• Leave multitiles out of the core.
• Better specify the use cases for multitiles
• Need to limit the “volume” of the requests

[joanma747]

Default request is too demanding. Takes everything!!. Need for assincronous

[aaime]

Asynchronous is most welcomed. One could also leave synchronous for formats that can be just streamed out, one tile at a time (e.g. a zip file full of tiles, a multipart). But for common formats, say geopackage, mbtiles, then yes, asynch is a must.