use guava's cache as implementation for the HotEntryCache

My own implementation was faster, but was not able to
implement a size limitation.

pdb-utils/build.gradle

@@ -1,5 +1,6 @@
 
 dependencies {
+    compile lib_guava
 	compile lib_log4j2_core
     compile lib_log4j2_slf4j_impl
 }