[Paranoia] Make EDSM plugin aware of any API limits

[Athanasius]

I don't think the player API on EDSM actually implements rate limiting.

Presumably if Anthor ever did it would be the same 'leaky bucket' he uses on the non-player endpoints.

So, out of paranoia, let's make the EDSM plugin aware of those headers and do something about them, even if it's only log if the bucket gets near empty.

i.e. on the systems API:

curl --verbose 'https://www.edsm.net/api-v1/systems?systemName=Sol'
...
< x-rate-limit-limit: 360
< x-rate-limit-remaining: 359
< x-rate-limit-reset: 10
...

[Athanasius]

As part of this we can also make this work more like the Inara API code in its plugin, but instead of a straight up timer we pay attention to the limits instead, meaning in many cases we could just send events immediately.

[Rixxan]

Took a whack at this one in 0c2134c. Will need to test this when I'm back from holiday to make sure it didn't break all the things.