[index] Create new TxIndex class.

The TxIndex will be responsible for building the transaction index
concurrently with the main validation thread by implementing
ValidationInterface. This does not process blocks concurrently yet.

src/Makefile.am

@@ -103,6 +103,7 @@ BITCOIN_CORE_H = \
   fs.h \
   httprpc.h \
   httpserver.h \
+  index/txindex.h \
   indirectmap.h \
   init.h \
   interfaces/handler.h \
@@ -204,6 +205,7 @@ libbitcoin_server_a_SOURCES = \
   consensus/tx_verify.cpp \
   httprpc.cpp \
   httpserver.cpp \
+  index/txindex.cpp \
   init.cpp \
   dbwrapper.cpp \
   merkleblock.cpp \