InnoDB could return the same list again and again if the buffer
passed to trx_recover_for_mysql() is smaller than the number of
transactions that InnoDB recovered in XA PREPARE state.

We introduce the transaction state TRX_PREPARED_RECOVERED, which
is like TRX_PREPARED, but will be set during trx_recover_for_mysql()
so that each transaction will only be returned once.

Because init_server_components() is invoking ha_recover() twice,
we must reset the state of the transactions back to TRX_PREPARED
after returning the complete list, so that repeated traversals
will see the complete list again, instead of seeing an empty list.
Without this tweak, the test main.tc_heuristic_recover would hang
in MariaDB 10.1.