Avoid deadlock and cancel dialog when using conditional formats

When a conditional format is used and the table does not load all the rows
at once, a deadlock ocurred when scrolling to a not yet loaded area.

This avoids the deadlock by unlocking before the conditional format query
is performed.

Additionally the cancel-query dialog is avoided when the threading mode of
SQLite is Serialize, since in that case the queries can be sent to SQLite
concurrently.

See discussion in PR #1503

src/sqlitedb.cpp

@@ -988,10 +988,14 @@ bool DBBrowserDB::executeMultiSQL(const QString& statement, bool dirty, bool log
 
 QByteArray DBBrowserDB::querySingleValueFromDb(const QString& sql, bool log)
 {
-    waitForDbRelease();
     if(!_db)
         return QByteArray();
 
+    // If the threading mode is Single-thread or Multi-thread then wait for
+    // the DB release.
+    if (sqlite3_db_mutex(_db) == nullptr)
+        waitForDbRelease();
+
     if(log)
         logSQL(sql, kLogMsg_App);
 

src/sqlitetablemodel.cpp

@@ -314,6 +314,8 @@ QVariant SqliteTableModel::data(const QModelIndex &index, int role) const
                 else
                     sql = QString("SELECT '%1' %2").arg(value, eachCondFormat.sqlCondition());
 
+                // Unlock before querying from DB
+                lock.unlock();
                 if (m_db.querySingleValueFromDb(sql, false) == "1")
                     return eachCondFormat.color();
             }