Keep the list of to-be-NOTIFYed names in a plain List palloc'd in

Keep the list of to-be-NOTIFYed names in a plain List palloc'd in
TopTransactionContext, rather than using Dllist.  This simplifies and
speeds up the code, and eliminates a former risk of coredump when
out of memory (since the old code didn't bother to check for malloc
failure).  It also moves us one step closer to retiring Dllist...