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/testdata/keystore/README

@@ -5,9 +5,9 @@ The "good" key files which are supposed to be loadable are:
 
 - File: UTC--2016-03-22T12-57-55.920751759Z--7ef5a6135f1fd6a02593eedc869c6d41d934aef8
   Address: 0x7ef5a6135f1fd6a02593eedc869c6d41d934aef8
-- File: UTC--2016-03-23T09-30-22.528630983Z--f466859ead1932d743d622cb74fc058882e8648a
+- File: aaa
   Address: 0xf466859ead1932d743d622cb74fc058882e8648a
-- File: UTC--2016-03-23T09-30-26.532308523Z--289d485d9771714cce91d3393d764e1311907acc
+- File: zzz
   Address: 0x289d485d9771714cce91d3393d764e1311907acc
 
 The other files (including this README) are broken in various ways

accounts/testdata/very-light-scrypt.json

@@ -0,0 +1 @@
+{"address":"45dea0fb0bba44f4fcf290bba71fd57d7117cbb8","crypto":{"cipher":"aes-128-ctr","ciphertext":"b87781948a1befd247bff51ef4063f716cf6c2d3481163e9a8f42e1f9bb74145","cipherparams":{"iv":"dc4926b48a105133d2f16b96833abf1e"},"kdf":"scrypt","kdfparams":{"dklen":32,"n":2,"p":1,"r":8,"salt":"004244bbdc51cadda545b1cfa43cff9ed2ae88e08c61f1479dbb45410722f8f0"},"mac":"39990c1684557447940d4c69e06b1b82b2aceacb43f284df65c956daf3046b85"},"id":"ce541d8d-c79b-40f8-9f8c-20f59616faba","version":3}