Add UserFunctionCache to cache UDF's library (#453)

* Add UserFunctionCache to cache UDF's library

This patch replace LibCache with UserFunctionCache. LibCache use HDFS
URL to identify a UDF's Library, and when BE process restart all of
downloaded library should be loaded another time. We use function id
corresponding to a library, and when process restart, all downloaded
libraries can be loaded without another downloading.

* update

be/test/runtime/test_data/user_function_cache/lib/my_add.cc

@@ -0,0 +1,4 @@
+void my_add() {
+}
+void my_del() {
+}