Cyclic replication networks

[krasserm]

For example, given three locations A, B and C, it should be able to connect them like:

 A --- C
  \   /
    B

With a replication network that may contain cycles, a location failure doesn't necessarily lead to a partition any more. Also, if the replication connection between A and B is broken, for example, due to a network failure, A and B can continue to exchange events via C.

So far, event replication recorded a replication trace (= position history) to drive event replication in a replicated event log. Replication traces will not be recorded any more and replication will be based on causality criteria (and additional custom replication filters if configured by an application). This is an important prerequisite for #106, #107 and #142.

Replication based on causality criteria in cyclic replication networks still ensures that local event logs are valid linear extensions of the happened-before relation of events.

[krasserm]

Closed with 9a2d4e5