Added classes Statement, Statement_map
Merge commit


sql/slave.cc:
  no THD::init_for_queries() as mem_root is statement-local
sql/sql_class.cc:
  added Statement
sql/sql_class.h:
  Statement added
sql/sql_parse.cc:
  no THD::init_for_queries()
sql/sql_prepare.cc:
  current_stmt_id -> statement_id_counter