Fix a couple of bugs in recent parallelism-related commits.

Commit 816e336f added the wrong error
check to async.c; sending restrictions is restricted to the leader,
not altogether unsafe.

Commit 3bd909b2 added ExecShutdownNode
to traverse the planstate tree and call shutdown functions, but made
a Gather node, the only node that actually has such a function, abort
the tree traversal, which is wrong.