[WIP] Add timestamp of the last transaction to addresses in ./wallet-tool.py display

[inaltoasinistra]

I think that it is useful provide more information in wallet display tool.
The test for BitcoinCoreInterface.get_lasts_txs_info is missed yet.

Example of output:

  m/0/4/1/1159 1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx used 0.90000000 btc 08 Nov 16 34:08
  m/0/4/1/1168 1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx used 0.23445421 btc 03 Dec 16 12:45
  m/0/4/1/1169 1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx used 3.31756245 btc 29 Nov 16 08:23
  m/0/4/1/1183 1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx used 2.42763463 btc 03 Jan 17 09:20

In the displayall command it shows exposed instead of used for old addresses which are not actually used.
Example:

  m/0/4/1/1158 1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx exposed 0.00000000 btc

[coveralls]

Coverage decreased (-0.4%) to 82.272% when pulling dffcea1 on the9ull:display-info into 05b758c on JoinMarket-Org:develop.

[coveralls]

Coverage decreased (-5.5%) to 77.235% when pulling b5f5d17 on the9ull:display-info into 05b758c on JoinMarket-Org:develop.