Prevent threads from respawning during joining

Summary:
Previously the thread pool might be non-empty after joining since concurrent submissions could spawn new threads. This problem didn't affect our background flush/compaction thread pools because the `shutting_down_` flag prevented new jobs from being submitted during/after joining. But I wanted to be able to reuse the `ThreadPool` without such external synchronization.
Closes https://github.com/facebook/rocksdb/pull/2953

Differential Revision: D5951920

Pulled By: ajkr

fbshipit-source-id: 0efec7d0056d36d1338367da75e8b0c089bbc973

util/threadpool_imp.cc

@@ -147,6 +147,9 @@ void ThreadPoolImpl::Impl::JoinThreads(bool wait_for_jobs_to_complete) {
 
   wait_for_jobs_to_complete_ = wait_for_jobs_to_complete;
   exit_all_threads_ = true;
+  // prevent threads from being recreated right after they're joined, in case
+  // the user is concurrently submitting jobs.
+  total_threads_limit_ = 0;
 
   lock.unlock();