Wire in bootstrap package for praefect for zero downtime deploy

fixes: #1898 (closed)

This MR refactors the praefect startup code to tie into bootstrap and includes some other changes:

• refactored cmd/gitaly/starter_configs.go to its own package internal/starter since both praefect and gitaly can share the code
• made a minor adjustments to the GracefulStoppableServer interface in internal/bootstrap so we can also treat praefect as a GracefulStoppableServer