Add getBlockTime interface method in src/interfaces/chain.h

- This method would allow to get the blocktime value of the block at
  the given height.

Github-Pull: #220
Rebased-From: 1fce853

Author: shaavan

src/interfaces/chain.h

@@ -139,6 +139,9 @@ class Chain
     //! Get block hash. Height must be valid or this function will abort.
     virtual uint256 getBlockHash(int height) = 0;
 
+    //! Get block time, Height must be valid or this function will abort.
+    virtual int64_t getBlockTime(int height) = 0;
+
     //! Check that the block is available on disk (i.e. has not been
     //! pruned), and contains transactions.
     virtual bool haveBlockOnDisk(int height) = 0;

src/node/interfaces.cpp

@@ -555,6 +555,11 @@ class ChainImpl : public Chain
         LOCK(::cs_main);
         return Assert(chainman().ActiveChain()[height])->GetBlockHash();
     }
+    int64_t getBlockTime(int height) override
+    {
+        LOCK(::cs_main);
+        return Assert(chainman().ActiveChain()[height])->GetBlockTime();
+    }
     bool haveBlockOnDisk(int height) override
     {
         LOCK(::cs_main);