get rid of node-fetch dependency

[sunify]

Why?

[troggy]

It is an extra dependency needed only for a very niche feature (fast exit). E.g. we are using leap-core in many scripts just for crunching data and have to drag node-fetch as well.

Probably, the better solution would be to extract exit market maker things in a separate package

On a bright side, node-fetch has no dependencies itself 🎉, so maybe not a big deal.

[sunify]

https://bundlephobia.com/result?p=node-fetch@2.6.0 441B in a bundle for browsers 🤷‍♂

[troggy]

best package ever :)