Add a benchmark for calling ConnectBlock on a block with many inputs

Requires placing block-107134.tar.gz (containing the block, and a fake CoinsDB
containing its inputs) into the base directory of the repository. This can be
generated using qa/zcash/create_benchmark_archive.py (see the script for usage
details).

To facilitate generation of the fake CoinsDB, an additional field 'valueZat' has
been added to 'getrawtransaction' containing the integer number of zatoshis
instead of a decimal number of ZEC.

Closes #2355.

src/txdb.h

@@ -31,6 +31,7 @@ class CCoinsViewDB : public CCoinsView
 {
 protected:
     CLevelDBWrapper db;
+    CCoinsViewDB(std::string dbName, size_t nCacheSize, bool fMemory = false, bool fWipe = false);
 public:
     CCoinsViewDB(size_t nCacheSize, bool fMemory = false, bool fWipe = false);