Previously, the last processor ('headProc') would be removed from
f.processors before flow.startInternal to be run in the same goroutine
as the one that is doing the setup. However, this was problematic
because if that processor implements 'Releasable' interface, it will not
be returned to the pool on the flow clean up. Now this is fixed.

Release note: None

Previously, new structs for rowBasedFlow and vectorizedFlow would be
allocated upon creation. This commit creates pools for both of them.
flowinfra.Releasable interface is moved into execinfra package because
now components from rowflow, rowexec, and colflow packages implement
that.

In order to actually be able to release the flow structs, I needed to
create separate Cleanup methods (which still share most of the logic)
which allows for removal of vectorized memory monitoring logic from
the shared FlowCtx.

Release note: None