document the need for local sessions within the readonly mode

[Hinterwaeldlers]

Starting with commit https://github.com/apache/zookeeper/commit/c47ef905e077184bc5b7f555a3e2dfeb6dc046e1 the readonly mode requires local sessions to be activated. This commit appends the related information to the docu.

In addition the impossibility to a client downgrade is noted

[eolivelli]

what happens if you do not enable them ?

[Hinterwaeldlers]

what happens if you do not enable them ?

The server is not serving any requests

I consider changing the behaviour within the code. But haven't found a sweet spot to activate the local session handling.

A different solution would be to change the behaviour of the session validation: https://github.com/apache/zookeeper/blob/c74658d398cdc1d207aa296cb6e20de00faec03e/zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/ReadOnlyZooKeeperServer.java#L122

In addition I am not sure, if Local-Sessions should be activated in regular mode or if there might be a scenario, where this leads to an unexpected behaviour

[maoling]

@Hinterwaeldlers Thanks for your contribution.