Transaction timing track problems when read-only trxs are involved

[linh2931]

Two problems were identified with transaction timing tracking when read-only trxs were involved:

• Parallelize Read-only Transaction Execution #776 assumed read-only transactions to only run on read-only threads. [4.0] Execute read only safe tasks on read-only thread pool #901 changed that and read-only transactions can run on main thread too.
  Timing summary for read-only transactions was based on when running on main thread, no other read-only transactions are running on read-only threads. This is no longer true.
• Idle time was left out being tracked due to a code refactoring by [4.0] Execute read only safe tasks on read-only thread pool #901

Close #930 for better problem tracking.

[linh2931]

Fixed by #947