Moved BlockchainDB into its own src/ subfolder

Ostensibly janitorial work, but should be more relevant later down the
line.  Things that depend on core cryptonote things (i.e.
    cryptonote_core) don't necessarily depend on BlockchainDB and thus
have no need to have BlockchainDB baked in with them.

src/cryptonote_core/CMakeLists.txt

@@ -30,8 +30,6 @@ set(cryptonote_core_sources
   account.cpp
   blockchain_storage.cpp
   blockchain.cpp
-  blockchain_db.cpp
-  BlockchainDB_impl/db_lmdb.cpp
   checkpoints.cpp
   checkpoints_create.cpp
   cryptonote_basic_impl.cpp
@@ -49,8 +47,6 @@ set(cryptonote_core_private_headers
   blockchain_storage.h
   blockchain_storage_boost_serialization.h
   blockchain.h
-  blockchain_db.h
-  BlockchainDB_impl/db_lmdb.h
   checkpoints.h
   checkpoints_create.h
   connection_context.h
@@ -76,6 +72,7 @@ target_link_libraries(cryptonote_core
   LINK_PUBLIC
     common
     crypto
+    blockchain_db
     ${Boost_DATE_TIME_LIBRARY}
     ${Boost_PROGRAM_OPTIONS_LIBRARY}
     ${Boost_SERIALIZATION_LIBRARY}
@@ -83,5 +80,4 @@ target_link_libraries(cryptonote_core
     ${Boost_FILESYSTEM_LIBRARY}
     ${Boost_SYSTEM_LIBRARY}
     ${Boost_THREAD_LIBRARY}
-    ${LMDB_LIBRARY}
     ${EXTRA_LIBRARIES})