clean up ephemeral nodes

[pH14]

I think there are two broad pieces here, neither being too crazy:

1. we need to write the code to clean up ephemeral nodes
2. we need to figure out how we want to choose a client to execute (1)

For the latter piece, there were a few suggestions in https://forums.foundationdb.org/t/fdb-zk-rough-cut-of-zookeeper-api-layer/1278 to build out a simple leader election, or a cooperative clock and electing a given client to do the work for a given period of time. I think it could also make sense to optionally offload some of this work onto a dedicated client, so that app clients aren't cycling through unrelated background work.

[pH14]

We can track ephemeral nodes in a separate subspace: (ephemeral_nodes, session_id, zkPath)

When we identify a session that has expired, we can range clear (ephemeral_nodes, session_id, *)

[pH14]

#13 resolves this