rpc: add next to getmininginfo

Obtain difficulty and target for the next block without having to call
getblocktemplate.

test/functional/mining_basic.py

@@ -213,6 +213,12 @@ class MiningTest(BitcoinTestFramework):
         assert_equal(mining_info['bits'], nbits_str(REGTEST_N_BITS))
         assert_equal(mining_info['target'], target_str(REGTEST_TARGET))
         assert_equal(mining_info['difficulty'], Decimal('4.656542373906925E-10'))
+        assert_equal(mining_info['next'], {
+            'height': 201,
+            'target': target_str(REGTEST_TARGET),
+            'bits': nbits_str(REGTEST_N_BITS),
+            'difficulty': Decimal('4.656542373906925E-10')
+        })
         assert_equal(mining_info['networkhashps'], Decimal('0.003333333333333334'))
         assert_equal(mining_info['pooledtx'], 0)