TokenBridge Contracts 6.0.0

Merge the develop branch to the master branch, preparation to v6.0.0

This update for the `master` branch contains the changes made to address findings discovered during a security audit:
 * [Fix] Stricter preconditions for payInterest (#623)
 * [Fix] Fix offset in comments (#624)
 * [Fix] Use fixed lower call gas limit (#627)
 * [Fix] Separate XDaiForeignBridge contract with compound and GSN support (#626)
 * [Fix] Update GSN interface (#628)
 * [Fix] Block ERC20 selectors in AMB requests (#630)
 * [Other] Bump package and contracts interfaces version prior to 6.0.0 (#629)