Added correct thread naming to compactor threads

[ddanielr]

Sets the thread name prefix based on the compaction type instead of defaulting to MajC.

I looked at using the existing CompactionType enum but it's in the public API.

accumulo/core/src/main/java/org/apache/accumulo/core/client/admin/ActiveCompaction.java

Lines 30 to 50 in cf57e34

	public abstract class ActiveCompaction {
	public static enum CompactionType {
	/**
	* compaction to flush a tablets memory
	*/
	MINOR,
	/**
	* Accumulo no longer does merging minor compactions.
	*/
	@Deprecated(since = "2.1.0", forRemoval = true)
	MERGE,
	/**
	* compaction that merges a subset of a tablets files into one file
	*/
	MAJOR,
	/**
	* compaction that merges all of a tablets files into one file
	*/
	FULL
	}

The goal is to name the MinC and MajC threads correctly for troubleshooting.