dummyHook's fields were concurrently written by nodes and read by
the test. The simplest solution is to protect all fields with a mutex.
Enable: TestMultiplePeersDropSelf, TestMultiplePeersDropOther as they
seemingly accidentally stayed disabled during a refactor/rewrite
since 1836366ac19e30f157570e61342fae53bc6c8a57.
resolvesethersphere/go-ethereum#1286