Auto merge of #2598 - str4d:2074-dbwrapper, r=str4d

Bitcoin 0.12+ dbwrapper improvements

Cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#6650
  - Only refactor - excludes obfuscation
- bitcoin/bitcoin#6777
  - Excluding obfuscation-related changes
- bitcoin/bitcoin#6865
- bitcoin/bitcoin#6823
- bitcoin/bitcoin#6873
- bitcoin/bitcoin#7927
  - Excluding first commit (already included) and second commit (obfuscation-related)
- bitcoin/bitcoin#8467

Part of #2074.

src/Makefile.am

@@ -152,7 +152,7 @@ BITCOIN_CORE_H = \
   init.h \
   key.h \
   keystore.h \
-  leveldbwrapper.h \
+  dbwrapper.h \
   limitedmap.h \
   main.h \
   memusage.h \
@@ -244,7 +244,7 @@ libbitcoin_server_a_SOURCES = \
   httprpc.cpp \
   httpserver.cpp \
   init.cpp \
-  leveldbwrapper.cpp \
+  dbwrapper.cpp \
   main.cpp \
   merkleblock.cpp \
   metrics.cpp \