Set VERSION_DISCOVERY_QUEUE_SIZE upstream.

[nuke-web3]

As I understand it, VERSION_DISCOVERY_QUEUE_SIZE is set directly above here and the comment is left to users to understand that this applies to this type in Polkadot. The Mocks and test uses this pattern: https://github.com/paritytech/polkadot/search?q=VERSION_DISCOVERY_QUEUE_SIZE

@bkchr Are you suggesting (from the PR comment) a default is set in the pallet itself here https://github.com/paritytech/polkadot/blob/master/xcm/pallet-xcm/src/lib.rs#L121 ?
If so, how do we expose this to end users and imply it can (and when it should) be overridden?

Originally posted by @nukemandan in #690 (comment)

[nuke-web3]

Overkill - not needed 😅