Add debug string and ODesc support to HashKey class

This allows tracing of hash key buffer reservations, reads, and writes via a new
debug stream, and supports printing a summary of a HashKey object via
Describe(). The latter comes in handy e.g. in TableVal::Describe() (where
including the hash key is now available but commented out).

src/DebugLogger.h

@@ -54,6 +54,7 @@ enum DebugStream
 	DBG_BROKER, // Broker communication
 	DBG_SCRIPTS, // Script initialization
 	DBG_SUPERVISOR, // Process supervisor
+	DBG_HASHKEY, // HashKey buffers
 
 	NUM_DBGS // Has to be last
 	};