Extract locking from main Cache class to LockManager and LockStoreInterface.
fix Cache::get(): return value even if not value object
Extract ttl (time-to-live handling) to separate class.
phpdoc for cache()
Simplify handling of non-metaphore cache value.
Add event dispatcher to allow adding listener for NO_STALE_CACHE special case.
Merge branch 'cache_method_refactoring'

Conflicts:
	src/Cache.php
Merge branch 'on_no_stale_cache'