[STORE] Synchronize operations that modify file mappings on DistributorDirectory

The rename(String, String) method doesn't allow this implementation to use a simple
concurrent map. There is a race during a rename operation where files are not fully
renamed but already visible via #listAll(). This inconsistency can lead to problems
when opening commit points since the pending_segments_N as well as segments_N are visible
but not yet atomically renamed.

Yet, non of the methods that are synced are long running such that adding sychronization
doesn't introduce bottlenecks here. The Direcotry#sync(...) method is not synchronized since
it doesn't change any mapping nor does it depend on the mapping.

Conflicts:
	src/main/java/org/elasticsearch/index/store/DistributorDirectory.java
	src/test/java/org/elasticsearch/index/store/DistributorDirectoryTest.java