Gsql prepared statements

Audit list
 - [ ] GSQLBackend
 - [ ] SSql
 - [x] gMySQL
 - [ ] gPgSQL
 - [ ] gSqlite3
 - [ ] gOracle
 - [ ] BIND (dnssec)
 - [ ] mydns
if (d_rrtable.empty()) { throw PDNSException("Records table must not be empty"); }