core, core/vm: added gas price variance table

This implements 1b & 1c of EIP150 by adding a new GasTable which must be
returned from the RuleSet config method. This table is used to determine
the gas prices for the current epoch.

Please note that when the CreateBySuicide gas price is set it is assumed
that we're in the new epoch phase.

In addition this PR will serve as temporary basis while refactorisation
in being done in the EVM64 PR, which will substentially overhaul the gas
price code.

cmd/utils/flags.go

@@ -792,6 +792,13 @@ func MakeChainConfigFromDb(ctx *cli.Context, db ethdb.Database) *core.ChainConfi
 		}
 		config.DAOForkSupport = true
 	}
+	if config.HomesteadGasRepriceBlock == nil {
+		if ctx.GlobalBool(TestNetFlag.Name) {
+			config.HomesteadGasRepriceBlock = params.TestNetHomesteadGasRepriceBlock
+		} else {
+			config.HomesteadGasRepriceBlock = params.MainNetHomesteadGasRepriceBlock
+		}
+	}
 	// Force override any existing configs if explicitly requested
 	switch {
 	case ctx.GlobalBool(SupportDAOFork.Name):