Use hash of key as priority. Vendor xxhash v0.8.2

VersionedMap.cpp

@@ -7,6 +7,7 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <unordered_set>
+#include <xxhash.h>
 
 void *mmapSafe(void *addr, size_t len, int prot, int flags, int fd,
                off_t offset) {
@@ -86,7 +87,7 @@ struct Entry {
     e->keyLen = keyLen;
     e->valLen = valLen;
     e->refCount = 1;
-    e->priority = rand(); // TODO
+    e->priority = XXH3_64bits(key, keyLen);
     e->clearTo = clearTo;
     memcpy((uint8_t *)e->getKey(), key, keyLen);
     ((uint8_t *)e->getKey())[keyLen] = 0;