* Change --gas=0 semantic in order to enable gas auto estimate.
* REST clients have been modified to simulate the execution of
the tx first to then populate the context with the estimated
gas amount returned by the simulation.
* The simulation returns both an unadjusted gas estimate and an
adjusted one. The adjustment is required to ensure that the
ensuing execution doesn't fail due to state changes that might
have occurred. Gas adjustment can be controlled via the CLI's
--gas-adjustment flag.
* Tiny refactorig of REST endpoints error handling.
Closes: #1246
