Makes a graph database give a helpful message when it's been shutdown

 * Removed throws IOException from NeoStoreXaDataSource constructor, because it's not true
 * Made ThreadToStatementContextBridge implement Lifecycle so it's told when to shutdown
 * Kernel keeps track of shutdown state and throws appropriate exception when used after
Merge pull request #758 from systay/databaseshutdownexception

Makes a graph database give a helpful message when it's been shutdown