MXS-1992 Implement cache size control

The cache now enforces the defined maximum size by evicting some
entries in case the insertion of a new entry would cause the max
size to be exceeded. Currently the eviction algorithm simply
removes a random element.

include/maxscale/query_classifier.h

@@ -435,13 +435,25 @@ typedef struct query_classifier
 } QUERY_CLASSIFIER;
 
 /**
- * QC_CACHE specifies the limits of the query classification cache.
+ * QC_CACHE_PROPERTIES specifies the limits of the query classification cache.
  */
 typedef struct QC_CACHE_PROPERTIES
 {
     int64_t max_size;  /** The maximum size of the cache. */
 } QC_CACHE_PROPERTIES;
 
+/**
+ * QC_CACHE_STATS provides statistics of the cache.
+ */
+typedef struct QC_CACHE_STATS
+{
+    int64_t size;      /** The current size of the cache. */
+    int64_t inserts;   /** The number of inserts. */
+    int64_t hits;      /** The number of hits. */
+    int64_t misses;    /** The number of misses. */
+    int64_t evictions; /** The number of evictions. */
+} QC_CACHE_STATS;
+
 /**
  * Loads and sets up the default query classifier.
  *
@@ -850,4 +862,13 @@ void qc_set_server_version(uint64_t version);
  */
 uint64_t qc_get_server_version();
 
+/**
+ * Get cache statistics for the calling thread.
+ *
+ * @param stats[out]  Cache statistics.
+ *
+ * @return True, if caching is enabled, false otherwise.
+ */
+bool qc_get_cache_stats(QC_CACHE_STATS* stats);
+
 MXS_END_DECLS