Previously multiple threads were allowed to load statistics concurrently.
There were no known problems caused by this. But given amount of data
races in this code, it'd happen sooner or later.

To avoid scalability bottleneck, statistics loading is protected by
per-TABLE_SHARE atomic variable.

Whenever statistics were loaded by preceding statement (hot-path), a
scalable load-acquire check is performed.

Whenever statistics have to be loaded anew, mutual exclusion for loaders
is established by atomic variable. If statistics are being loaded
concurrently, statement waits until load is completed.

TABLE_STATISTICS_CB::stats_can_be_read and
TABLE_STATISTICS_CB::stats_is_read are replaced with a tri state atomic
variable.

Part of MDEV-19061 - table_share used for reading statistical tables is
                     not protected