1. Refactored innobase_close_connection(). Transaction must've already
been rolled back by this time. We should expect only transactions in the
PREPARED state when MDEV-742 is done.

2. Added missing put_pins() to trx_disconnect_prepared(). Missing
put_pins() wasn't a problem because trx_disconnect_prepared() is a dead
code. But it will get revived in the main MDEV-742 patch.

3. Fixed missing reset of trx->mysql_log_file_name when RW transaction
didn't emit any log records (zero-modification RW). The problem was
detected by ASAN when disconnected XA transaction was trying to make
use of inherited mysql_log_file_name pointing into binlog data of
detached THD.

This missing reset also had user-visible side effect, when
trx_sys_print_mysql_binlog_offset() would report binlog position
not of the most recently committed transaction.

One of possible scenarios that is expected to misbehave is as following:

  thr1> CREATE TABLE t1(a INT) ENGINE=InnoDB;
  thr1> INSERT INTO t1 VALUES(1);
  thr1> BEGIN;
  thr1> UPDATE t1 SET a=1
  thr1> COMMIT; -- zero-modification, misses to reset mysql_log_file_name

  thr2> BEGIN;
  thr2> INSERT INTO t1 VALUES(2);
  thr2> COMMIT;

  thr1> BEGIN;
  thr1> do-some-real-changes;
  thr1> ROLLBACK; -- will store binlog pos from previous COMMIT in thr1?

In this case it means if binlog is replayed from position reported by
trx_sys_print_mysql_binlog_offset(), t1 will end up with two records
containing '2'.

Part of
MDEV-742 - XA PREPAREd transaction survive disconnect/server restart