Implement MigrationService

The MigrationService allows us to perform necessary migrations when
upgrading to a new Bisq version. So far, the only option is to try to
perform the migrations on each run. The MigrationService runs the
necessary migrations if the data directory version is below the app
version.

application/src/main/java/bisq/application/migration/MigrationService.java

@@ -0,0 +1,46 @@
+package bisq.application.migration;
+
+import bisq.common.application.ApplicationVersion;
+import bisq.common.platform.Version;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.concurrent.CompletableFuture;
+
+public class MigrationService {
+    static final Version VERSION_BEFORE_MIGRATION_SERVICE_INTRODUCED = new Version("2.1.1");
+    private final Path dataDir;
+    private final File dataDirVersionFile;
+
+    public MigrationService(Path dataDir) {
+        this.dataDir = dataDir;
+        this.dataDirVersionFile = dataDir.resolve("version").toFile();
+    }
+
+    public CompletableFuture<Boolean> runMigrations() {
+        Version dataDirVersion = getDataDirVersion();
+        Version appVersion = ApplicationVersion.getVersion();
+
+        if (dataDirVersion.below(appVersion)) {
+            Migrator migrator = new Migrator(appVersion, dataDir);
+            migrator.migrate();
+        }
+
+        return CompletableFuture.completedFuture(true);
+    }
+
+    Version getDataDirVersion() {
+        if (!dataDirVersionFile.exists()) {
+            return VERSION_BEFORE_MIGRATION_SERVICE_INTRODUCED;
+        }
+
+        try {
+            String version = Files.readString(dataDirVersionFile.toPath());
+            return new Version(version);
+        } catch (IOException e) {
+            throw new RuntimeException("Can't identify data dir version. This shouldn't happen.", e);
+        }
+    }
+}

application/src/main/java/bisq/application/migration/Migrator.java

@@ -0,0 +1,32 @@
+package bisq.application.migration;
+
+import bisq.application.migration.migrations.Migration;
+import bisq.common.platform.Version;
+import lombok.extern.slf4j.Slf4j;
+
+import java.nio.file.Path;
+import java.util.Collections;
+import java.util.List;
+
+@Slf4j
+public class Migrator {
+    private final Version appVersion;
+    private final Path dataDir;
+    private final List<Migration> allMigrations;
+
+    public Migrator(Version appVersion, Path dataDir) {
+        this.appVersion = appVersion;
+        this.dataDir = dataDir;
+        this.allMigrations = Collections.emptyList();
+    }
+
+    public void migrate() {
+        for (Migration migration : allMigrations) {
+            Version migrationVersion = migration.getVersion();
+            if (migrationVersion.belowOrEqual(appVersion)) {
+                log.info("Running {} migrations.", migrationVersion);
+                migration.run(dataDir);
+            }
+        }
+    }
+}

application/src/main/java/bisq/application/migration/migrations/Migration.java

@@ -0,0 +1,11 @@
+package bisq.application.migration.migrations;
+
+import bisq.common.platform.Version;
+
+import java.nio.file.Path;
+
+public interface Migration {
+    void run(Path dataDir);
+
+    Version getVersion();
+}