A feature to skip compilation of SecretRegistry

[pirapira]

When we upgrade the Solidity compiler #1155 but want to keep an old compilation of SecretRegistry, we need a feature where contracts except SecretRegistry is compiled.

This issue keeps track of implementing such a partial compilation.

[pirapira]

One way of doing it: make compile_contracts does not compile SecretRegistry, and checks that the source code of SecretRegistry did not change.

[pirapira]

I need to figure out how to detect source changes https://github.com/raiden-network/raiden-contracts/issues/1257.