This fixes a regression caused by MDEV-12428.
When we introduced a variant of fil_space_acquire() that could
increment space->n_pending_ops after space->stop_new_ops was set,
the logic of fil_check_pending_operations() was broken.

fil_space_t::n_pending_ios: A new field to track read or write
access from the buffer pool routines immediately before a block
write or after a block read in the file system.

fil_space_acquire_for_io(), fil_space_release_for_io(): Similar
to fil_space_acquire_silent() and fil_space_release(), but
modify fil_space_t::n_pending_ios instead of fil_space_t::n_pending_ops.

fil_space_free_low(): Wait for space->n_pending_ios to reach 0,
to avoid accessing freed data in a concurrent thread. Future
calls to fil_space_acquire_for_io() will not find this tablespace,
because it will already have been detached from fil_system.

Adjust a number of places accordingly, and remove some redundant
tablespace lookups.

FIXME: buf_page_check_corrupt() should take a tablespace from
fil_space_acquire_for_io() as a parameter. This will be done
in the 10.1 version of this patch and merged from there.
That depends on MDEV-12253, which has not been merged from 10.1 yet.