simplification of peerset api (#2123)

* Introduction of PeersetHandle

* integrate PeersetHandle with the rest of the codebase

* fix compilation errors

* more tests for peerset, fixed overwriting bug in add_reserved_peer

* Slots data structure and bugfixes for peerset

* bend to pressure

* updated lru-cache to 0.1.2 and updated linked-hash-map to 0.5.2

* peerset discovered list is now a LinkedHashMap

* fix review suggestions

* split back Peerset and PeersetHandle

* test for Peerset::discovered

* applied review suggestions

* fixes to peerset::incoming

* peerset disconnects are all instantaneous

* instantaneous drop in peerset finished

* Peerset::set_reserved_only can also reconnect nodes

* Peerset scores cache uses lru-cache

* remove redundant function call and comment from Peerset::on_set_reserved_only

* add_peer returns SlotState enum

* apply review suggestions

* is_reserved -> is_connected_and_reserved