FLUME-752: Refactor HBase sinks to re-use common code/functionality

Review Request #1900 — Created Aug. 22, 2011 and updated

As agreed in comments at https://review.cloudera.org/r/1847/ this is the part of patch dedicated solely to refactoring of HBase sinks.

New base AbstractHBaseSink was extracted which holds common functionality (common sink attributes which are basically HBase client attributes and HTable connection management code: open, close, etc.)

  1. Alex, looks good.  Please update the license and post patch to the jira.  I'll commit then (no need for another review).
  2. change to apache license:
     * Copyright 2011 The Apache Software Foundation
     * Licensed to the Apache Software Foundation (ASF) under one
     * or more contributor license agreements.  See the NOTICE file
     * distributed with this work for additional information
     * regarding copyright ownership.  The ASF licenses this file
     * to you under the Apache License, Version 2.0 (the
     * "License"); you may not use this file except in compliance
     * with the License.  You may obtain a copy of the License at
     *     http://www.apache.org/licenses/LICENSE-2.0
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
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     * See the License for the specific language governing permissions and
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