refactoring LockService to extract interface

LockService has been renamed to LockServiceImpl and LockService is now an
interface.
Refactoring code base to access LockServiceImpl through interface

This is the initial commit for LockServiceFactory. Previously a LockService was
obtained through the static method LockService.getInstance. LockService has
been refactored into an interface and the getInstance method has been moved to
LockServiceFactory.getLockService.
Adding registry to LockServiceFactory

LockServiceFactory has been refactored to store an internal registry of
LockService instances. Similar to other liquibase factories, it uses
ServiceLocator to find LockService classes.

LockService has been refactored to extend PrioritizedService.
fixing test compiler errors caused from previous refactoring
Merge branch 'extensible-lock-service' of https://github.com/jsanda/liquibase