Add txrequest module

This adds a new module (unused for now) which defines TxRequestTracker, a data
structure that maintains all information about transaction requests, and coordinates
requests.

src/uint256.cpp

@@ -80,4 +80,5 @@ template std::string base_blob<256>::ToString() const;
 template void base_blob<256>::SetHex(const char*);
 template void base_blob<256>::SetHex(const std::string&);
 
+const uint256 uint256::ZERO(0);
 const uint256 uint256::ONE(1);