Refactoring of 'starter' package and gitaly server factory

Extending package 'starter' with 'Parse', 'Compose' funcs to work with gitaly address format. 'GracefulStoppableServer' is a common interface that is why it has been moved to 'starter' package where it is used. 'GitalyServerFactory' moved into 'server' package as 'bootstrap' is used by praefect as well. Also it now supports multiple servers to be started for listening and track all of them correctly. All changes covered with tests.

This is another step after !2293 (merged) before !2276 (merged).