Estimate gas price adequately

[matthiasgeihs]

Copied from hyperledger-labs/go-perun#332

Location

https://github.com/hyperledger-labs/go-perun/blob/9349d86249e080d3fc93d8d5ac82197a0c07f0d0/backend/ethereum/channel/contractbackend.go#L38

Problem

We just set a default, very high gasLimit for all calls.

Proposal

Set a more appropriate gas limit per call.