Implement PostgresSchedDB::repackExists()

Implement missing method to check if repack job exists.

Feb  1 17:05:58.831944 ctafrontend cta-frontend: LVL="INFO" PID="333" TID="369" MSG="In RequestMessage::process(): Admin command succeeded." user="ctaadmin2@ctafrontend" command="virtualorganization" subcommand="ch" status="failure" reason="Not implemented /lib64/libctacommon.so.0(cta::exception::Backtrace::Backtrace(bool)+0x6b) [0x7fdf9ab8aa9d] /lib64/libctacommon.so.0(cta::exception::Exception::Exception(std::basic_string_view<char, std::char_traits<char> >, bool)+0x91) [0x7fdf9ab8bab5] /lib64/libctascheduler.so.0(cta::PostgresSchedDB::repackExists()+0x5c) [0x7fdf9c5b9dba] /lib64/libctascheduler.so.0(cta::Scheduler::repackExists()+0x3c) [0x7fdf9c50ab96] /lib64/libXrdSsiCta.so(cta::frontend::AdminCmd::processVirtualOrganization_Ch(cta::xrd::Response&)+0x39e) [0x7fdf9ca46e4a] /lib64/libXrdSsiCta.so(cta::frontend::AdminCmd::process()+0xbcb) [0x7fdf9ca3d693] /lib64/libXrdSsiCta.so(cta::xrd::RequestMessage::process(cta::xrd::Request const&, cta::xrd::Response&, XrdSsiStream*&)+0x18b) [0x7fdf9c98dea5] /lib64/libXrdSsiCta.so(XrdSsiPb::RequestProc<cta::xrd::Request, cta::xrd::Response, cta::xrd::Alert>::ExecuteAction()+0x157) [0x7fdf9c98bc3f] /lib64/libXrdSsiCta.so(XrdSsiPb::RequestProc<cta::xrd::Request, cta::xrd::Response, cta::xrd::Alert>::Execute()+0xd0) [0x7fdf9c989028] /lib64/libXrdSsiCta.so(XrdSsiPb::Service<cta::xrd::Request, cta::xrd::Response, cta::xrd::Alert>::ProcessRequest(XrdSsiRequest&, XrdSsiResource&)+0x8f) [0x7fdf9c987eff] /lib64/libXrdUtils.so.3(XrdScheduler::Run()+0x13a) [0x7fdfa540233a] /lib64/libXrdUtils.so.3(XrdStartWorking(void*)+0x9) [0x7fdfa5402459] /lib64/libXrdUtils.so.3(XrdSysThread_Xeq+0x37) [0x7fdfa538e727] /lib64/libpthread.so.0(+0x7ea5) [0x7fdfa44f6ea5] /lib64/libc.so.6(clone+0x6d) [0x7fdfa421fb0d]" VO="vo_cta" WRITE_MAX_DRIVES="2" MAX_FILE_SIZE="100" IS_REPACK_VO="false" adminTime="0.004635"