Problem #1:

The function row_vers_impl_x_locked_low() wrongly identifies a transaction to
hold an implicit x lock on a secondary index, which is orphaned.  Orphaned
means that the secondary index is delete marked and purgeable and does not have
its associated clustered index record.  

Solution #1:

Added an extra check in row_vers_impl_x_locked_low() to detect this situation
and confirm that the transaction does not hold an implicit x lock on the
record.

Problem #2:

The active transaction was removed from trx_sys->rw_trx_set prematurely.

Solution #2:

The trx was removed from trx_sys->rw_trx_set only after the trx state change
and release of all trx locks.

rb#4291 approved by Marko, Yasufumi, Sunny.