Introducing ledger shards and new ledger syncing logic. (#247)

- The ledgers are stored in a sqlite database in ledger file system.
- Ledgers are organized in shards inside primary folder.
- Raw inputs are saved as shards inside blob folder. No input files are created if neither inputs nor outputs are available.
- Both last primary shard hash and last blob shard hashes are subjected to consensus and necessary sync operations are performed if out of sync.
- Hpfs sync support setting single sync targets from both ends of the list. (Targets set from front are prioritized).
- Contract and ledger syncs and serving are performed independently.

src/usr/input_nonce_map.cpp

@@ -1,6 +1,6 @@
 #include "../pchheader.hpp"
 #include "../util/util.hpp"
-#include "../ledger.hpp"
+#include "../ledger/ledger.hpp"
 #include "input_nonce_map.hpp"
 
 namespace usr