Clean up the working of the main thread; add a tunable parameter srv_max_buf_pool_modified_pct which can be used to make the flush phase in shutdown quicker


innobase/srv/srv0srv.c:
  Clean up the working of the main thread; add a tunable parameter srv_max_buf_pool_modified_pct which can be used to make the flush phase in shutdown quicker
innobase/include/buf0buf.h:
  Clean up the working of the main thread; add a tunable parameter srv_max_buf_pool_modified_pct which can be used to make the flush phase in shutdown quicker
innobase/include/srv0srv.h:
  Clean up the working of the main thread; add a tunable parameter srv_max_buf_pool_modified_pct which can be used to make the flush phase in shutdown quicker
innobase/buf/buf0buf.c:
  Clean up the working of the main thread; add a tunable parameter srv_max_buf_pool_modified_pct which can be used to make the flush phase in shutdown quicker