Prevent djmp from spawning a mapproxy instance on every request

[mvanveen]

@simod comments:

right now djmp is spawning mapproxy instance on any call to /pk/map/x. This means that for every ogc requests, any single tile, we spawn a new mapproxy that will die immediately.
Would it make sense and be possible to keep it alive for some time and share it among requests?

here is the offending line of code.

@simod suggests:

we could store the configured mp to at least avoid the configuration all the times

[mvanveen]

udpate: @ingenieroariel has explained to me that there's a lot of perf considerations and trade-offs with both approaches depending on whether we're doing simple proxying or users are doing a lot of heavy rendering with no caching. So, there's definitely some nuance around this proposed modification and if it makes sense for all use cases.