Added RunnersRegistry, an API to register and, later, run a group of actions.

Added RunnersRegistry, an API to register and, later, run a group of actions.

Useful for keeping general initialization/cleanup management code (e.g.,
main.cc) independent from specific initialization/cleanup code (e.g.,
Store file systems or memory cache) during staged initialization and
cleaning.

Designed with Rock Store needs in mind. Currently unused. Should eventually be
used for most modules initialization and cleanup, removing main.cc dependency
on those modules and perfecting [de]initialization order.