innodb_max_purge_lag_wait_update(): Return immediately if we are
in high_level_read_only mode.

srv_wake_purge_thread_if_not_active(): Relax a debug assertion.
If srv_read_only_mode holds, purge_sys.enabled() will not hold
and this function will do nothing.

trx_t::commit_in_memory(): Remove a redundant condition before
invoking srv_wake_purge_thread_if_not_active().