Ensure variable values are quoted when necessary while sending down SET

Ensure variable values are quoted when necessary while sending down SET
comamnds to the remote nodes

Earlier we'd special cased a few GUCs such as those using memory or time units
or transaction isolation levels. But clearly that wasn't enough as we noticed
with "application_name" recently. So fix this problem in a more comprehensive
manner.

Added a few more test cases to cover these scenarios.