trx_t::is_recovered: Revert most of the changes that were made by the
merge of MDEV-15326 from 10.2. The trx_sys.rw_trx_hash and the recovery
of transactions at startup is quite different in 10.3.

trx_free_at_shutdown(): Avoid excessive mutex protection. Reading fields
that can only be modified by the current thread (owning the transaction)
can be done outside mutex.

trx_t::commit_state(): Restore a tighter assertion.

trx_rollback_recovered(): Clarify why there is no potential race condition
with other transactions.

lock_trx_release_locks(): Merge with trx_t::release_locks(),
and avoid holding lock_sys.mutex unnecessarily long.

rw_trx_hash_t::find(): Remove redundant code, and avoid starving the
committer by checking trx_t::state before trx_t::reference().