Reduce gas estimate multiplier

[tombeynon]

REStake currently applies a flat 1.5x multiplier to the simulated gas estimate across all chains. This was used because it was stable across all chains but I haven't tested if it can be reduced in some time. I imagine the default can be reduced for all chains as the implementation has improved.

We can also make this customisable per chain for autostaking, so operators can reduce their gas costs. Would allow operators to tweak their networks until they find their ideal gas setting. Ideally we also have optimised defaults per chain in networks.json.

Relates to #480