This PR reimplements the light client server pool. It is also a first step
to move certain logic into a new lespay package. This package will contain
the implementation of the lespay token sale functions, the token buying and
selling logic and other components related to peer selection/prioritization
and service quality evaluation. Over the long term this package will be
reusable for incentivizing future protocols.
Since the LES peer logic is now based on enode.Iterator, it can now use
DNS-based fallback discovery to find servers.
This document describes the function of the new components:
https://gist.github.com/zsfelfoldi/3c7ace895234b7b345ab4f71dab102d4
* les: move execqueue into utilities package
execqueue is a util for executing queued functions
in a serial order which is used by both les server
and les client. Move it to common package.
* les: move randselect to utilities package
weighted_random_selector is a helpful tool for randomly select
items maintained in a set but based on the item weight.
It's used anywhere is LES package, mainly by les client but will
be used in les server with very high chance. So move it into a
common package as the second step for les separation.
* les: rename to utils
