Gsql prepared statements

Audit list
 - [ ] GSQLBackend
 - [ ] SSql
 - [x] gMySQL
 - [ ] gPgSQL
 - [ ] gSqlite3
 - [ ] gOracle
 - [ ] BIND (dnssec)
 - [ ] mydns
d_rrtable, d_soatable, d_soawhere, d_rrwhere are use just inside MyDNSBackend constructor. There is no need to have then as attributes.