Account for sampling level in key estimating fns
encapsulate utility methods in Downsampling class and bikeshed docstrings

Conflicts:

	src/java/org/apache/cassandra/io/sstable/IndexSummary.java
	src/java/org/apache/cassandra/io/sstable/IndexSummaryBuilder.java
	src/java/org/apache/cassandra/io/sstable/IndexSummaryManager.java
	src/java/org/apache/cassandra/io/sstable/SSTableReader.java
	test/unit/org/apache/cassandra/io/sstable/IndexSummaryTest.java
	test/unit/org/apache/cassandra/io/sstable/SSTableReaderTest.java