wallet: add dummy BerkeleyRODatabase and BerkeleyROBatch classes

BerkeleyRODatabase and BerkeleyROBatch will be used to access a BDB file
without the use of BDB. For now, these are dummy classes.

src/Makefile.am

@@ -347,6 +347,7 @@ BITCOIN_CORE_H = \
   wallet/feebumper.h \
   wallet/fees.h \
   wallet/load.h \
+  wallet/migrate.h \
   wallet/receive.h \
   wallet/rpc/util.h \
   wallet/rpc/wallet.h \
@@ -507,6 +508,7 @@ libbitcoin_wallet_a_SOURCES = \
   wallet/fees.cpp \
   wallet/interfaces.cpp \
   wallet/load.cpp \
+  wallet/migrate.cpp \
   wallet/receive.cpp \
   wallet/rpc/addresses.cpp \
   wallet/rpc/backup.cpp \