Refactor GraphService, graph auto-scan and auto-reload.

This pull request is related to #1519. It also contains some other changes:

- Simplify `GraphService` class and eliminate the class hierarchy (`GraphServiceBeanImpl`, `GraphServiceAutoDiscoverImpl`, `GraphServiceFileImpl`, `GraphServiceImpl`...) with a simpler interface and a single default implementation. The responsability of `GraphService` is now simpler, previous capabilities (auto-scan, auto-reload, memory beans...) have been delegated to external classes.
- The GraphService now manage `GraphSource`s, instead of `Graph`s. A `GraphSource` is responsible for ownership, creation and reload of Graph from whatever source (for now: memory, serialized file on disk, on classpath).
- This splits the somehow unrelated auto-scan (automatically scanning for graph under a base directory) and auto-reload features (reloading a graph if the source has been modified).
- The auto-scan and initial startup graph registration is delegated to a `GraphScanner` class, external to the `GraphService`.
- For registering and saving graphs through the existing web-service API, a `GraphSourceFactory` is responsible of creating new `GraphSource` given a routerIds, and saving serialized graph data. This looks a bit convoluted but is necessary for backward compatibility of the graph registration API.

In non pre-evict mode, if a graph load fails, we keep the old version available. When replacing a new graph, the copy should be done in less than 10 seconds (parametrable). If the copy is not done in this time, one or more load will fails before succeding, but w/o impacting the requests. This only "pollutes" the log. For large graphs on production servers, an atomic move with backup is preferable anyway.

In pre-evict mode, user requests are blocked while loading a new graph (this is necessary as we want to remove the old graph before loading the new one to save memory). Currently pre-evict is not parametrable via command line parameters but this could be added quite easily.