MDL_CONTEXT::TRY_ACQUIRE_LOCK_IMPL

ANALYSIS:
=========
Server sometimes exited when multiple threads tried to
acquire and release metadata locks simultaneously (for
example, necessary to access a table). The same problem
could have occurred when new objects were registered/
deregistered in Performance Schema.

The problem was caused by a bug in LF_HASH - our lock free
hash implementation which is used by metadata locking
subsystem in 5.7 branch. In 5.5 and 5.6 we only use LF_HASH
in Performance Schema Instrumentation implementation. So
for these versions, the problem was limited to P_S.

The problem was in my_lfind() function, which searches for
the specific hash element by going through the elements
list. During this search it loads information about element
checked such as key pointer and hash value into local
variables. Then it confirms that they are not corrupted by
concurrent delete operation (which will set pointer to 0)
by checking if element is still in the list. The latter
check did not take into account that compiler (and
processor) can reorder reads in such a way that load of key
pointer will happen after it, making result of the check
invalid.

FIX:
====
This patch fixes the problem by ensuring that no such
reordering can take place. This is achieved by using
my_atomic_loadptr() which contains compiler and processor
memory barriers for the check mentioned above and other
similar places.

The default (for non-Windows systems) implementation of
my_atomic*() relies on old __sync intrisics and implements
my_atomic_loadptr() as read-modify operation. To avoid
scalability/performance penalty associated with addition of
my_atomic_loadptr()'s we change the my_atomic*() to use
newer __atomic intrisics when available. This new default
implementation doesn't have such a drawback.