Use Ivy for resolving external dependencies.

Includes an upgrade to the latest metrics-base commit 256e619.

Implements part of #31326.

CHANGELOG.md

+# Changes in version 2.7.0 - 2019-0?-??
+
+ * Medium changes
+   - Use Ivy for resolving external dependencies rather than relying
+     on files found in Debian stable packages. Requires installing Ivy
+     (using `apt-get install ivy`, `brew install ivy`, or similar) and
+     running `ant resolve` (or `ant -lib /usr/share/java resolve`).
+     Retrieved files are then copied to the `lib/` directory, except
+     for dependencies on other metrics libraries that still need to be
+     copied to the `lib/` directory manually. Current dependency
+     versions resolved by Ivy are the same as in Debian stretch with
+     few exceptions.
+
+
 # Changes in version 2.6.2 - 2019-05-29
 
  * Medium changes

build.xml

@@ -4,7 +4,8 @@
        <!ENTITY base SYSTEM "src/build/java/base.xml">
 ]>
 
-<project default="usage" name="metrics-lib" basedir=".">
+<project default="usage" name="metrics-lib" basedir="."
+         xmlns:ivy="antlib:org.apache.ivy.ant">
 
   <property name="release.version" value="2.6.2-dev" />
   <property name="javadoc-title" value="Tor Metrics Library API Documentation"/>

build @ 256e6192

-Subproject commit ed4819215aa60cc8935f955d9404f30e83dd892b
+Subproject commit 256e6192ad0066e48c6abb0ee9ac71714c7a3a0f