Add history feature

This change adds an in-memory storage for emitted log lines organised by topics. Additionally history is understood by clients now and manifests in a protocol addition.

Every line is stored in the history which organises the topics into buckets. Those buckets are fixed-size arrays acting as ring buffers. Over the `chan HistoryRequest` clients can request the history of a topic returning lines up to the configured bucket size.

The protocol needs more formalisation and should potentially go into sub-package for re-use in clients implementations. It could be even more hardened by moving it into visor itself and share the protocol definition.

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The commits in this branch will be cleaned up in time and likely stashed to a single coherent change-set
/rfr @bernerdschaefer @grobie @juliusv