Perform individual decryptions inside a database transaction.

Required a lot of random locking work to prevent deadlocking, but
overall this results in about a 2x speed increase for decryptions.

app/src/main/java/org/thoughtcrime/securesms/database/DatabaseLock.java

@@ -0,0 +1,32 @@
+package org.thoughtcrime.securesms.database;
+
+import android.content.Context;
+
+import androidx.annotation.NonNull;
+
+import net.sqlcipher.database.SQLiteDatabase;
+
+import java.io.Closeable;
+
+public final class DatabaseLock {
+
+  public static @NonNull Lock acquire(@NonNull Context context) {
+    SQLiteDatabase db = DatabaseFactory.getInstance(context).getRawDatabase();
+
+    if (db.isDbLockedByCurrentThread()) {
+      return () -> {};
+    }
+
+    db.beginTransaction();
+
+    return () -> {
+      db.setTransactionSuccessful();
+      db.endTransaction();
+    };
+  }
+
+  public interface Lock extends Closeable {
+    @Override
+    void close();
+  }
+}