TopologyCache is a collection of deserialized structures with
information about cluster topology. This currently includes data from
_pico_instance, _pico_replicaset, _pico_tier & _pico_service_route_table.
The info is automatically kept up to date with the corresponding system
tables. The TopologyCache also caches the immutable info related to the
current instance, like instance name, replicaset uuid, etc.

From now on we should be reading this data from TopologyCache whenever
possible instead of going directly to system tables as we were doing
previosly.

At the moment only the plugin RPC module has transitioned to using
TopologyCache but other modules should be refactored as well.
Especially governor.