use TabletWriter::Thread to load/unload/split TabletIO

[00k]

Now we use thread_pool to load/unload/split. Because of the limit of thread number, we have to control the concurrency. So when a ts restart, the load process is very slow. Every TabletIO has a TabletWriter, every TabletWriter has a thread. We could use that thread to do these work.

[taocp]

we always need a concurrency control?

[00k]

Yes. But the thread number will not be out limitation any more. The concurrency could be very large, e.g. 100.

[caijieming-ng]

why not TabletWriter shared in thread pool?