Support pallet instances in bridge-relayers pallet

[svyatonik]

After #1654 we are identifying "bridge" in bridge-relayers pallet with LaneId. But it isn't consistent with out other pallets. The current approach is 1 grandpa + messages pallet for single bridged chain (parachains is a bit different - it supports all parachains of the bridged relay chain). And since we don't support instances in the relayers pallet, it would mean that it may only be used by single bridge (unless we make LaneId values unique across all bridges).

The solutions are:

• add instances support to the bridge-relayers pallet;
• use pair (ChainId, LaneId) to identify payment source.
  It isn't actual now, since we're planning to have only one instance of every pallet initially

[serban300]

Resolved by #1907