Prepare for 2.12.1 release.

CHANGELOG.md

-# Changes in version 2.??.? - 2020-??-??
+# Changes in version 2.12.1 - 2020-04-30
 
  * Minor changes
    - Change back how we treat xz-compressed files by leaving

build.xml

@@ -7,7 +7,7 @@
 <project default="usage" name="metrics-lib" basedir="."
          xmlns:ivy="antlib:org.apache.ivy.ant">
 
-  <property name="release.version" value="2.12.0-dev" />
+  <property name="release.version" value="2.12.1" />
   <property name="javadoc-title" value="Tor Metrics Library API Documentation"/>
   <property name="javadoc-excludes" value="**/impl/** **/index/** **/internal/** **/log/**" />
   <property name="implementation-title" value="Tor Metrics Library" />