Deprecate HTablePool in favor of HConnection.getTable(...)

This issue introduces a few new APIs: * HConnectionManager: {code}     public static HConnection createConnection(Configuration conf)     public static HConnection createConnection(Configuration conf, ExecutorService pool) {code} * HConnection: {code}     public HTableInterface getTable(String tableName) throws IOException     public HTableInterface getTable(byte[] tableName) throws IOException     public HTableInterface getTable(String tableName, ExecutorService pool) throws IOException     public HTableInterface getTable(byte[] tableName, ExecutorService pool) throws IOException {code} By default HConnectionImplementation will create an ExecutorService when needed. The ExecutorService can optionally passed be passed in. HTableInterfaces are retrieved from the HConnection. By default the HConnection's ExecutorService is used, but optionally that can be overridden for each HTable.