Signed-off-by: NKenan Yao <kyao@pivotal.io>

QD need to cancel QEs when
1) QD get a error
2) one or more QEs got error and cancelOnError was set to true.

We want to cancel QEs as soon as possible once above conditions are reached, but considering
the cost of cancelling QEs is high, we want to process as many pending finish QEs as possible
before actually cancel. The original interval before cancelling is 2 seconds which is too
long that users will see an obvious delay before errors are reported, this commit lower
this interval to 100 ms to speed up the cancelling process.

refactor CdbDispatchUtilityStatement() to make it flexible for cdbCopyStart(),
dispatchVacuum() to call directly. Introduce flags like DF_NEED_TWO_SNAPSHOT,
DF_WITH_SNAPSHOT, DF_CANCEL_ON_ERROR to make function call much clearer

dispatcher/ directory

This commit has no logic change, it just contains movement of code across
files, to make dispatcher code clearer, and easier for unit testing.

Signed-off-by: Kenan Yao

refactor cdbdisp_dispatchToGang interface.
refactor memory management in dispatch.

Remove unnecessary #includes, add #includes that are actually needed by
some headers.

SET command is session effective, all existed idle gangs should be set for later reuse,
but for busy gangs declared by cursors, errors occur if they receive a set command. Way
to fix it is marking busy gangs to no reuse so they can be destroyed after cursors been
closed.