prune-trace: add SamplingPruner

The SamplingPruner implements "normal" sampling with equivalence-class
reuse.  Unlike the FESamplingPruner, the SamplingPruner implements
uniform fault-space sampling that counts multiple hits of an
equivalence class.

This change modifies the database schema, more specifically it adds
the "weight" column to the fspgroup table.  Update existing databases
with this query:

  ALTER TABLE fspgroup ADD COLUMN weight INT UNSIGNED;

Change-Id: I668fc9b25fc4d79a60aa1ef8d69cdf5fa076cc6d

tools/prune-trace/FESamplingPruner.hpp

@@ -30,7 +30,6 @@ public:
 
 	void getAliases(std::deque<std::string> *aliases) {
 		aliases->push_back("FESamplingPruner");
-		aliases->push_back("sampling");
 	}
 
 private: