Fix ref counting race between SSTableScanner and SSTR

Patch by jmckenzie; reviewed by marcuse for CASSANDRA-8399
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/compaction/AbstractCompactionStrategy.java
	src/java/org/apache/cassandra/db/compaction/CompactionIterable.java
	src/java/org/apache/cassandra/db/compaction/CompactionManager.java
	src/java/org/apache/cassandra/db/compaction/LeveledCompactionStrategy.java
	src/java/org/apache/cassandra/db/compaction/WrappingCompactionStrategy.java
	src/java/org/apache/cassandra/io/sstable/format/SSTableReader.java
	src/java/org/apache/cassandra/io/sstable/format/big/BigTableScanner.java
	src/java/org/apache/cassandra/tools/SSTableExport.java
	test/unit/org/apache/cassandra/db/compaction/AntiCompactionTest.java
	test/unit/org/apache/cassandra/db/compaction/CompactionsTest.java
	test/unit/org/apache/cassandra/db/compaction/LeveledCompactionStrategyTest.java
	test/unit/org/apache/cassandra/db/compaction/TTLExpiryTest.java
	test/unit/org/apache/cassandra/io/sstable/SSTableReaderTest.java
	test/unit/org/apache/cassandra/io/sstable/SSTableRewriterTest.java
	test/unit/org/apache/cassandra/io/sstable/SSTableScannerTest.java
	test/unit/org/apache/cassandra/io/sstable/SSTableUtils.java
add missing file
Merge branch 'cassandra-2.1' into trunk