Mark ItemPointer arguments as const in tuple/table lock functions

Mark ItemPointer arguments as const in tuple/table lock functions

The functions LockTuple, ConditionalLockTuple, UnlockTuple, and
XactLockTableWait take an ItemPointer argument that they do not
modify, so the argument can be const-qualified to better convey intent
and allow the compiler to enforce immutability.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAEoWx2m9e4rECHBwpRE4%2BGCH%2BpbYZXLh2f4rB1Du5hDfKug%2BOg%40mail.gmail.com