make sure the shutdown thread calling to datawriters isn't interrupted

SCATTER-128 may lead or relate to GDA-5479 MXGDA-985
BLXXJ-27 test if an empty message is sent during configure and wait for
a proper response.

It has been seen that when da.server is first connected to after it has
been restarted then it may sent out an empty message ('>') before
sending out the response to the startup commands. 

This can get things out of sync and mess up the GDA configure process.
merge from I20-1-prod to 8.26
Merge remote-tracking branch 'origin/8.26'