Qsync perf optimizations

This ticket is a set of optimizations which could speed up the synchronous transaction, and reduce slow down of the async transactions.

• txn_journal_entry_new() in each statement dereferences stmt->space->def->opts in order to check whether the space is sync. This is 3 pointer derefences and could be quite slow;
• txn_commit() has 3 checks for whether the transaction is sync: it takes TXN_WAIT_SYNC flags, + it checks is_sync boolean variable twice. First way to reduce number of checks - check is_sync only once and then handle async and sync transactions completely separately. Second way - keep track of sync space count in the schema. When it is 0, don't even try to check any sync flags.
• <Add more optimizatons here>.

Start by locating txn_journal_entry_new() and txn_commit(), then trace how stmt->space->def->opts, TXN_WAIT_SYNC, and is_sync are used for synchronous and asynchronous transactions. The issue leaves additional optimizations open; done means the selected changes preserve transaction behavior while reducing the relevant sync checks and improving the stated performance.