* change type of db_descriptors to SegmentDatabaseDescriptor **

A new gang definination may consist of cached segdbDesc and new
created segdbDesc, there is no need to palloc all segdbDesc struct
as new.

* Remove unnecessary allocate gang unit test

* Manage idle segment dbs using CdbComponentDatabases instead of available* lists.

To support vary size gang, we now need to manage segment dbs in a lower
granularity, previously, idle QEs is managed by a bunch of lists like
availablePrimaryWriterGang, availableReaderGangsN, this restrict
dispatcher to only create N-size (N = number of segments) or 1-size
gang.

CdbComponentDatabases is a snapshot of segment components within current
cluster, now it maintains a freelist for each segment component. When
creating gang, dispatcher will make up a gang from each segment
component (from freelist or create a new segment db). When cleaning up
a gang, dispatcher will return idle segment dbs to each segment
component.

CdbComponentDatabases provide a few functions to manipulate segment dbs
(SegmentDatabaseDescriptor *):
* cdbcomponent_getCdbComponents
* cdbcomponent_destroyCdbComponents
* cdbcomponent_allocateIdleSegdb
* cdbcomponent_recycleIdleSegdb
* cdbcomponent_cleanupIdleSegdbs

CdbComponentDatabases is also FTS version sensitive, so once a FTS
version changed, CdbComponentDatabases destroy all idle segment dbs
and allocate QEs in the new promoted segment. This provides the ability
to transparent mirror failover to users.

Since segment dbs(SegmentDatabaseDescriptor *) are managed by
CdbComponentDatabases now, we can simplify the memory context
management by replacing GangContext & perGangContext with
DispatcherContext & CdbComponentsContext.

* Postpone the error hanlding when creating gang

Now we have AtAbort_DispatcherState, one advantage of it is that
we can postpone gang error hanlding in this function and make
code cleaner.

* Handle FTS version change correctly

In some cases, when a FTS version changed, we can't update current
snapshot of segment components, to be more specifically, we can't
destroy current writer segment dbs and create new segment dbs.

These cases include:
* session has temp table created.
* query need two-phase commit and gxid has been dispatched to
  segments.

* Replace <gangId, sliceId> map with <qeIdentifier, sliceId> map

We used to dispatch a <gangId, sliceId> map along with query to
segment dbs so segment dbs can know which slice they should
execute.

Now gangId is useless for a segment db because a segment db can
be reused by different gang, so we need a new way to tell the
info to segment dbs. To resolve this, CdbComponentDatabases
assign a unique identifier to each segment db and make up a
bitmap set which consist of segment identifiers for each slice,
segment dbs then can go through the slice table and find the
right slice to execute.

* Allow dispatcher to create vary size gang and refine AssignGangs()

Previously, dispatcher can only create N-size gang for
GANGTYPE_PRIMARY_WRITER or GANGTYPE_PRIMARY_READER. this
restrict dispatcher in many ways, one example is direct
dispatch, it always create a N-size gang even it only
dispatch the command to one segment, another example is
some operations may be able to use N+ size gang, like
hash join, if both inner and outer plan is redistributed,
the hash join node can associate with a N+ size gang to
execute. This commit changes the API of createGang() so the
caller can specify a list of segments (partial or even
duplicate segments), CdbCompoentDatabase will guarantee
each segment has only one writer in a session. With this
it also resolves another pain point of AssignGangs(), so
the caller don't need to promote a GANGTYPE_PRIMARY_READER
to GANGTYPE_PRIMARY_WRITER, or promote a GANGTYPE_SINGLETON
_READER to GANGTYPE_PRIMARY_WRITER for replicated table
(see FinalizeSliceTree()).

With this commit, AssignGang() is very clear now.