innobase/include/srv0srv.h:
  Add configuration parameter srv_max_purge_lag.
  Add global variable srv_dml_needed_delay.
innobase/include/trx0sys.h:
  Add trx_sys->rseg_history_len
innobase/row/row0mysql.c:
  Add row_mysql_delay_if_needed() for delaying INSERTs, UPDATEs and
  DELETEs for srv_dml_needed_delay microseconds.
innobase/srv/srv0srv.c:
  Define global variable srv_dml_needed_delay.
  Define configuration parameter srv_max_purge_lag.
innobase/trx/trx0purge.c:
  Update trx_sys->rseg_history_len.
  trx_purge(): Compute srv_dml_needed_delay from srv_max_purge_lag
  and trx_sys->rseg_history_len.
innobase/trx/trx0rseg.c:
  Initialize trx_sys->rseg_history_len at InnoDB start-up.
sql/ha_innodb.h:
  Add configuration parameter srv_max_purge_lag.
sql/mysqld.cc:
  Add startup option innodb_max_purge_lag,
  with default value 0 (meaning infinite, disabling the feature).
sql/set_var.cc:
  Add global variable innodb_max_purge_lag.