Bitcoin ABC already had such a constant, so this just renames it to use
the same as Core and uses it in p2p_compactblocks.py
This is a backport of Core PR18764 [2/3]
https://github.com/bitcoin/bitcoin/pull/18764/commits/b35e1d2471c3d9e0b6cc4b94f4726a7e6ea32206
Depends on D9141