accounts: cache key addresses

In order to avoid disk thrashing for Accounts and HasAccount,
address->key file mappings are now cached in memory. This makes it no
longer necessary to keep the key address in the file name. The address
of each key is derived from file content instead.

There are minor user-visible changes:

- "geth account list" now reports key file paths alongside the address.
- If multiple keys are present for an address, unlocking by address is
  not possible. Users are directed to remove the duplicate files
  instead. Unlocking by index is still possible.
- Key files are overwritten written in place when updating the password.

accounts/key_store_passphrase_test.go

@@ -17,16 +17,25 @@
 package accounts
 
 import (
+	"io/ioutil"
 	"testing"
 
 	"github.com/ethereum/go-ethereum/common"
 )
 
+const (
+	veryLightScryptN = 2
+	veryLightScryptP = 1
+)
+
 // Tests that a json key file can be decrypted and encrypted in multiple rounds.
 func TestKeyEncryptDecrypt(t *testing.T) {
-	address := common.HexToAddress("f626acac23772cbe04dd578bee681b06bdefb9fa")
-	keyjson := []byte("{\"address\":\"f626acac23772cbe04dd578bee681b06bdefb9fa\",\"crypto\":{\"cipher\":\"aes-128-ctr\",\"ciphertext\":\"1bcf0ab9b14459795ce59f63e63255ffd84dc38d31614a5a78e37144d7e4a17f\",\"cipherparams\":{\"iv\":\"df4c7e225ee2d81adef522013e3fbe24\"},\"kdf\":\"scrypt\",\"kdfparams\":{\"dklen\":32,\"n\":262144,\"p\":1,\"r\":8,\"salt\":\"2909a99dd2bfa7079a4b40991773b1083f8512c0c55b9b63402ab0e3dc8db8b3\"},\"mac\":\"4ecf6a4ad92ae2c016cb7c44abade74799480c3303eb024661270dfefdbc7510\"},\"id\":\"b4718210-9a30-4883-b8a6-dbdd08bd0ceb\",\"version\":3}")
+	keyjson, err := ioutil.ReadFile("testdata/very-light-scrypt.json")
+	if err != nil {
+		t.Fatal(err)
+	}
 	password := ""
+	address := common.HexToAddress("45dea0fb0bba44f4fcf290bba71fd57d7117cbb8")
 
 	// Do a few rounds of decryption and encryption
 	for i := 0; i < 3; i++ {
@@ -44,7 +53,7 @@ func TestKeyEncryptDecrypt(t *testing.T) {
 		}
 		// Recrypt with a new password and start over
 		password += "new data appended"
-		if keyjson, err = EncryptKey(key, password, LightScryptN, LightScryptP); err != nil {
+		if keyjson, err = EncryptKey(key, password, veryLightScryptN, veryLightScryptP); err != nil {
 			t.Errorf("test %d: failed to recrypt key %v", i, err)
 		}
 	}