"trancaction_count" instead of "time"

[gogo9th]

In the .xml configuration file, I suggest adding the new option of "transaction_count" as an optional replacement of "time". For people who uses BenchBase to evaluate database recovery, they need to do evaluations based on the window width of committed transactions (e.g., 1000 tx, 1M tx, 1B tx). For example, if <transaction_count>1000000</transaction_count>, BenchBase will terminate right after committing the 1M-th transaction. Therefore, adding the "transaction_count" feature will be useful for them. If this option is not offered, they need to implement this on their own. If you think this feature can be useful, I will create a PR for this.

[bpkroth]

Seems potentially useful, but it might be a little tricky given there could be multiple threads each issuing a different set of transactions, so coordinating the exact count between them might be expensive. I might suggest to batch it so that threads only communicate their executed batch count every so often and check whether the count has exceeded the threshold after each batch. There should be some infrastructure for this already.