core, eth, miner: improved tx removal & fatal error on db sync err

* core: Added GasPriceChange event
* eth: When one of the DB flush methods error a fatal error log message
  is given. Hopefully this will prevent corrupted databases from
  occuring.
* miner: remove transactions with low gas price. Closes #906, #903

core/events.go

@@ -1,8 +1,10 @@
 package core
 
 import (
-	"github.com/ethereum/go-ethereum/core/types"
+	"math/big"
+
 	"github.com/ethereum/go-ethereum/core/state"
+	"github.com/ethereum/go-ethereum/core/types"
 )
 
 // TxPreEvent is posted when a transaction enters the transaction pool.
@@ -44,6 +46,8 @@ type ChainUncleEvent struct {
 
 type ChainHeadEvent struct{ Block *types.Block }
 
+type GasPriceChanged struct{ Price *big.Int }
+
 // Mining operation events
 type StartMining struct{}
 type TopMining struct{}