Make it possible to rebuild a Maven project without changing the version

- synthesize the release value by incrementing the release of the previous successful build with the same name and version
 - add an extra level to the storage hierarchy for Maven builds so they can be differentiated by release
 - create the per-tag Maven repos by symlinking into the storage hierarchy, rather than copying files
 - significant refactoring of Maven build environment population and tracking to differentiate between builds of the same version

docs/schema.sql

@@ -613,8 +613,7 @@ CREATE TABLE maven_builds (
         build_id INTEGER NOT NULL PRIMARY KEY REFERENCES build(id),
 	group_id TEXT NOT NULL,
         artifact_id TEXT NOT NULL,
-        version TEXT NOT NULL,
-	CONSTRAINT maven_builds_un UNIQUE (group_id, artifact_id, version)
+        version TEXT NOT NULL
 ) WITHOUT OIDS;
 
 -- Even though we call this archiveinfo, we can probably use it for