Move LockClauseStrength, LockWaitPolicy into new file nodes/lockoptions.h.

Commit df630b0d moved enum LockWaitPolicy
into its very own header file utils/lockwaitpolicy.h, which does not seem
like a great idea from here.  First, it's still a node-related declaration,
and second, a file named like that can never sensibly be used for anything
else.  I do not think we want to encourage a one-typedef-per-header-file
approach.  The upcoming foreign table inheritance patch was doubling down
on this bad idea by moving enum LockClauseStrength into its *own*
can-never-be-used-for-anything-else file.  Instead, let's put them both in
a file named nodes/lockoptions.h.  (They do seem to need a separate header
file because we need them in both parsenodes.h and plannodes.h, and we
don't want either of those including the other.  Past practice might
suggest adding them to nodes/nodes.h, but they don't seem sufficiently
globally useful to justify that.)

Committed separately since there's no functional change here, just some
header-file refactoring.