[BUG] WalletConnect pairing cleanup

[jribbink]

Repeatedly connecting to an app can result in a large number of pairings being stored between the app and the wallet (if you repeatedly connect, you can look in the WalletConnect tab in Flow Wallet and see that there are a number of unused sessions).

This can be somewhat expensive in the WalletConnect protocol (theoretically, a very large number of pairings could put load on sign-client by opening a large number of subscriptions, so FCL should be able to either re-use or cleanup these pairings when duplicates are established).

Care must be taken to avoid destroying active pairings (i.e. be cognizant of any current/future use of concurrent sessions with the same wallet). This would suggest that the pairing re-use approach is probably more resilient than destroying existing pairings.

This was less of an issue in the previous iteration of FCL-WC's usage, as pairings were frequently re-used. However, in the new implementation with integrated QR codes, this problem is significantly amplified and should ideally be addressed before a production release (currently these changes are only in an alpha state).