* Simplify direct dispatch related code

This commit include two parts:
* simplify direct-dispatch dispatching code
* simplify direct-dispatch DTM related code

Previously, cdbdisp_dispatchToGang need a CdbDispatchDirectDesc info,
now gang only contain inuse segments, so direct-dispatch info is useless.

Another thing is, we need to decide if DTM is available for direct-dispatch
within dtmPreCommand, the logic is complex, you need to know if the main plan
is direct-dispatch and if the init plan contain direct-dispatch.

one example is:
"update foo set foo.c2 = 2
where foo.c1 = 1 and exists (select * from bar where bar.c1=4)"

main plan can be direct dispatched to segment 1, init plan can be direct
dispatched to segment 2, with the old logic, the DTM like PREPARE need to
dispatched to all segments, so dtmPreCommand need to dispatch a DTM named
'DTX_PROTOCOL_COMMAND_STAY_AT_OR_BECOME_IMPLIED' to all segment so those
segments like segment 3 who didn't receive the plan can be ready for two
phase commit.

With the new gang API, we can simplify this process, we add a list in
currentGxact to record which segments are actually get involved in a two
phase commit, then we can dispatch DTM to them directly. This is also very
usefully for queries on tables that are not fully expaned yet.

* support direct dispatch to more than one segment