While the index_lock and block_lock include debug instrumentation
to keep track of shared lock holders, such instrumentation was
never part of the simpler srw_lock, and therefore some users of the
class implemented a limited form of bookkeeping.

srw_lock_debug encapsulates srw_lock and adds the data members
writer, readers_lock, and readers to keep track of the threads that
hold the exclusive latch or any shared latches. The debug checks
are available also with SUX_LOCK_GENERIC (in environments that do not
implement a futex-like system call).

dict_sys_t::latch: Use srw_lock_debug in debug builds.
This makes the debug fields latch_ex, latch_readers redundant.

fil_space_t::latch: Use srw_lock_debug in debug builds.
This makes the debug field latch_count redundant.
The field latch_owner must be preserved, because
fil_space_t::is_owner() is being used in all builds.

lock_sys_t::latch: Use srw_lock_debug in debug builds.
This makes the debug fields writer, readers redundant.

lock_sys_t::is_holder(): A new debug predicate to check if
the current thread is holding lock_sys.latch in any mode.

trx_rseg_t::latch: Use srw_lock_debug in debug builds.