Utilize the solidity SMT Checker for our contracts

[LefterisJP]

The slides by @axic are here: https://drive.google.com/file/d/1AniKaHdhmDG3nkKP9jbyuE_-pjmFCl4n/view

In order to use the SMTChecker we need to add the following pragma:

pragma experimentalSMTChecker;

We should use it in order to have yet another way of detecting bugs that we may not have managed to spot on our own yet.

A nice video on explaining the usage of the SMTChecker is here.

[pirapira]

#566 is for TokenNetwork.

[karlb]

See the results at https://github.com/raiden-network/raiden-contracts/issues/566#issuecomment-596655928. We should give this another try at a later time.