Add a persistent screen showing basic node metrics

The screen is implemented using ANSI Escape sequences.

Closes #1331

src/Makefile.am

@@ -120,6 +120,7 @@ BITCOIN_CORE_H = \
   main.h \
   memusage.h \
   merkleblock.h \
+  metrics.h \
   miner.h \
   mruset.h \
   net.h \
@@ -205,6 +206,7 @@ libbitcoin_server_a_SOURCES = \
   leveldbwrapper.cpp \
   main.cpp \
   merkleblock.cpp \
+  metrics.cpp \
   miner.cpp \
   net.cpp \
   noui.cpp \