trx_set_rw_mode(): Check the flag high_level_read_only instead
of testing srv_force_recovery (innodb_force_recovery) directly.
There is no need to prevent the creation of read-write transactions
if innodb_force_recovery=3 is used. Yes, in that mode any recovered
incomplete transactions will not be rolled back, but these transactions
will continue to hold locks on the records that they have modified.
If the new read-write transactions hit conflicts with already existing
(possibly recovered) transactions, the lock wait timeout mechanism
will work just fine.