The bug could cause a crash when several connections needed
persistent statistics for the same table.
Also added a missing call of set_statistics_for_table() in the code
of the function mysql_update.
If a query referenced some system statistical tables, but not all of them,
then executing an ANALYZE command simultaneously with this query could
lead to a deadlock.
The fix prohibited reading statistics from system statistical tables
for such queries.
Removed the function unlock_tables_n_open_system_tables_for_write()
as not used anymore.
Performed some minor refactoring of the code in sql_statistics.cc.
engine-independent statistics.
If a table was created for InnoDB then the execution of the
ANALYZE command over this table blocked any INSERT/DELETE/UPDATE
of the table.
table_stat -> table_stats
column_stat -> column_stats
index_stat -> index_stats
to be in line with the names of innodb statistical tables
from mysql-5.6: innodb_table_stats and innodb_index_stats.
