Explicitly initialize StageManager early in startup

StageManager initializes itself through a static block and sets up every Stage, including pre-starting their threads. This initizalization can take a few hundred milliseconds. This timing impact is unpredictable and hard to reason about; it looks like it usually gets hit when creating new Keyspaces on start-up and announcing them through migrations.

While these processes are resilient to these delays, it is dramatically easier to reason about over time if this initialization happens explicitly.