Drop TreeRoutingTable

### Motivation
The `TreeRoutingTable` class is practically a relic; let's remove it from from the `RoutingTable` hierarchy.

### Changelog
* Move all useful code of `TreeRoutingTable` into `OptimizedTreeRoutingTable` and drop the redundant class.
* Update some docstrings and any failing tests.
* Drop setting of `failedRPC` attribute; attribute is unused.