Dump thread may encounter an error when reading events from the active binlog
file. However the errors may be temporary, so dump thread will try to read
the event again. But dump thread seeked to an wrong position, it caused some
events was sent twice.

To fix the bug, prev_pos is defined out the while loop and is set the correct
position after reading every event correctly.

This patch also make binlog_can_be_corrupted more accurate, only the binlogs
not closed normally are marked binlog_can_be_corrupted.

Finally, two warnings are added when dump threads encounter the temporary
errors.