Revert "sanlock: Shrink thread pool when there is no work"
    
    This reverts commit 0ff9c1ab8852bec846822ee2af55ebcb7e5f5967.
    
    This patch causes unexplained growth in memory usage.
    Part of the problem may be that the worker threads are
    not joined and the detached state is not set, but an
    initial test setting the detached state didn't seem
    to fix the problem.