[CURATOR-223] Add executorService methods to ServiceCacheBuilder

Add executorService methods to ServiceCacheBuilder to allow the caller to specify
an ExecutorService or a CloseableExecutorService to be used by the PathChildrenCache
embedded in ServiceCacheImpl.

Extracts ExecuteCalledWatchingExecutorService (and DelegatingExecutorService) into
the curator-test module for use by TestServiceCache.
7 files changed