Gsql prepared statements

Audit list
 - [ ] GSQLBackend
 - [ ] SSql
 - [x] gMySQL
 - [ ] gPgSQL
 - [ ] gSqlite3
 - [ ] gOracle
 - [ ] BIND (dnssec)
 - [ ] mydns
Same as gmysqlbackend: the if is not necessary since you can delete null pointers safely and it's possible to use a method to do both: delete and set to null.