combine the Trx threads into one thread

At trx coprocessor start time, it will start two threads, one for lease, for for old transaction cleanup. And there is a third memory monitor thread currently not start by default.

These threads are per region.

In one system, there are 200 regions, so we saw RegionServer has over 680 threads, and it cause a high CPU usage, and high System Time usage.

One improvement is we can combine the threads into one, since they both are periodic routines.