rpc: move gas estimator into core, dedup in simulated backend

[karalabe]

Currently we have 2 gas estimators. One in internals/ethapi, the other in the simulated backend. This was kind of acceptable when they were a few liners, but now we have some extra optimizations in place for the main one but which wasn't ported into the simulated one. Also it's not an exactly testable part of our code.

TL;DR: We should move the gas estimation logic into core, either into it's own package or along with something else and have the RPCs only call it.