Add java docs for StatsCollector constructor and StatsCollectorCallback thread safety

java/org/rocksdb/StatisticsCollectorCallback.java

@@ -7,6 +7,13 @@ package org.rocksdb;
 
 /**
  * Callback interface provided to StatisticsCollector.
+ * 
+ * Thread safety:
+ * StatisticsCollector doesn't make any guarantees about thread safety. 
+ * If the same reference of StatisticsCollectorCallback is passed to multiple
+ * StatisticsCollector references, then its the responsibility of the 
+ * user to make StatisticsCollectorCallback' implementation thread-safe.
+ * 
  * @param tickerType
  * @param tickerCount
 */