Add a mechanism for the dynamic update of partition-host mapping to the PartitionRouter (PartitionedMirrorClient)

[DawidNiezgodka]

Currently we fetch information about the partition-host mapping once during the initialisation of the PartitionRouter. Should the mapping change, for example because of the addition of another replica, some cache-misses will occur as there is no update-mechanism. Thus, it is advisable to introduce such a mechanism, i.e. the possibility to update the mapping dynamically.

A possible approach:

1. If there is a host-miss, the request is forwarded to the correct replica without any error,
2. In addition, a prompt to update the cache is written into a header,
3. The gateway uses this info to forward it to the router which reinits (updates) the mapping.