Lower transfer expiration for dApp

[palango]

In https://github.com/raiden-network/team/issues/894 and https://github.com/raiden-network/team/issues/891 we made the case for a shorter time between receiving a secret and the on-chain transaction in case we don't receive an unlock.

A decision has been made, and we need to implement it for the dApp (and possibly the CLI).

• SDK
  • Make lock expiration configurable
  • Add test for the minimal expiration
• dApp
  • Use lower expiration by default
• CLI
  • Decide if lower expirations should be used

[palango]

I read the code and tested it manually: The PC accepts mediations up until the reveal timeout, so no action on the python side is necessary.