MDEV-14203: rpl.rpl_extra_col_master_myisam, rpl.rpl_slave_load_tmpdir_not_exist failed in buildbot with a warning

Problem:
=======
rpl.rpl_slave_load_tmpdir_not_exist 'stmt' w3 [ fail ]  Found warnings/errors
in server log file!

Test ended at 2017-09-27 20:34:55
[Warning] Master is configured to log replication events with checksum, but
will not send such events to slaves that cannot process them
^ Found warnings in /mnt/buildbot/build/mariadb-10.2.10/mysql-test/var/3/log/mysqld.1.err
ok
Analysis:
========
When slave tries to connect to master 'get_master_version_and_clock' function
is invoked to perform elaborated slave-master handshake. During this process
slave server queries master server, to know if it is checksum aware and at the
same time master is notified about its CRC-awareness. The master's side
instant value of @@global.binlog_checksum is stored in the dump thread's
uservar area as well as cached locally to become known in consensus by master
and slave.

Post hand-shake slave requests master for binlog dump. It sends
'COM_BINLOG_DUMP'. This command is sent to master by 'cli_advanced_command'
call. If there is some temporary network failure during this request_dump
call, 'end_server' is invoked to close the current connection between master
and slave. Upon connection close the dump thread on the master gets terminated
and it clears the 'uservar' data it got through master-slave handshake.

The 'COM_BINLOG_DUMP' command is sent once again without master-slave
handshake. Since the checksum data is not available with new dump thread a
warning gets reported.

Fix:
===
Upon network write error donot attempt reconnect, proceed to master-slave
handshake. This ensures that master is aware of slave's capability to use
checksums.