Create if not exists

[ertankucukoglu]

Hello,

Latest snapshot builds includes "drop if exists" support (at least for tables).
Both "drop if exists" and "create if not exists" has been discussed in several other places including #4203
Since half of the job seems to be implemented, I would very much like to have the other half for v6.0.

Thank you for your consideration.

Regards,
Ertan