Include a map from sliceIndex to gang_id in the dispatched string,
and remove the localSlice field, hence QE should get the localSlice
from the map now. By this way, we avoid duplicating and modifying
the dispatch text string slice by slice, and each QE of a sliced
dispatch would get same contents now.

The extra space cost is sizeof(int) * SliceNumber bytes, and the extra
computing cost is iterating the SliceNumber-size array. Compared with
memcpy of text string for each slice in previous implementation, this
way is much cheaper, because SliceNumber is much smaller than the size
of dispatch text string. Also, since SliceNumber is so small, we just
use an array for the map instead of a hash table.

Also, clean up some dead code in dispatcher, including:
(1) Remove primary_gang_id field of Slice struct and DispatchCommandDtxProtocolParms
struct, since dispatch agent is deprecated now;
(2) Remove redundant logic in cdbdisp_dispatchX;
(3) Clean up buildGpDtxProtocolCommand;